BGP Functional Overview
BGP uses TCP as its transport protocol. Two BGP routers form a TCP connection between one another (peer routers) and exchange messages to open and confirm the connection parameters.
BGP routers exchange network reachability information. This information is mainly an indication of the full paths (BGP autonomous system numbers) that a route should take to reach the destination network. This information helps construct a graph that shows which autonomous systems are loop free and where routing policies can be applied to enforce restrictions on routing behavior.
Any two routers forming a TCP connection to exchange BGP routing information are called peers or neighbors. BGP peers initially exchange their full BGP routing tables. After this exchange, incremental updates are sent as the routing table changes. BGP keeps a version number of the BGP table, which is the same for all of its BGP peers. The version number changes whenever BGP updates the table due to routing information changes. Keepalive packets are sent to ensure that the connection is alive between the BGP peers and notification packets are sent in response to error or special conditions.
 Note |
VPNv4 address family is supported effective from Cisco IOS XR Release 6.1.31. However, VPNv6 and VPN routing and forwarding (VRF) address families will be supported in a future release. |
Prerequisites for Implementing BGP
The current Internet BGP table contains approximately 1.1 million IPv4 routes and 200,000 IPv6 routes. With an average of two paths per route, the BGP process typically requires around 5.5 GB of RAM to manage the full Internet BGP table. As the IPv6 Internet table continues to expand, the memory requirements for the BGP process are expected to increase. Therefore, Cisco recommends using the Service Edge (SE) version of Route Processor (RP) or Route Switch Processor (RSP) cards, or fixed chassis, on routers that will maintain a full BGP table
BGP Router Identifier
For BGP sessions between neighbors to be established, BGP must be assigned a router ID. The router ID is sent to BGP peers in the OPEN message when a BGP session is established.
BGP attempts to obtain a router ID in the following ways (in order of preference):
-
By means of the address configured using the bgp router-id command in router configuration mode.
-
By using the highest IPv4 address on a loopback interface in the system if the router is booted with saved loopback address configuration.
-
By using the primary IPv4 address of the first loopback address that gets configured if there are not any in the saved configuration.
If none of these methods for obtaining a router ID succeeds, BGP does not have a router ID and cannot establish any peering sessions with BGP neighbors. In such an instance, an error message is entered in the system log, and the show bgp summary command displays a router ID of 0.0.0.0. After BGP has obtained a router ID, it continues to use it even if a better router ID becomes available. This usage avoids unnecessary flapping for all BGP sessions. However, if the router ID currently in use becomes invalid (because the interface goes down or its configuration is changed), BGP selects a new router ID (using the rules described) and all established peering sessions are reset.
Note |
We strongly recommend that the bgp router-id command is configured to prevent unnecessary changes to the router ID (and consequent flapping of BGP sessions). |
BGP Route Distinguisher
In network design solutions where customer equipment is dual-homed and Fast Reroute is required, such as in EVPN and BGP PIC Edge solutions, the Route Distinguisher (RD) associated with each VRF must be unique per Provider Edge (PE) router. In other design scenarios, while it isn’t mandatory for the RD to be unique per PE, it is highly recommended to make it unique. This practice facilitates easier transitions to dual-homed solutions in the future.
There are few available options to keep unique RD per device:
-
Manual configuration: You must manually assign a unique value per device in the network. For example, in this scenario:
-
Leaf (ToR) = RD 1
-
Edge DCI Gateway = RD 2
-
Remote PE = RD 3
-
-
Use rd auto command under VRF. To assign a unique route distinguisher for each router, you must ensure that each router has a unique BGP router-id. If so, the rd auto command assigns a Type 1 route distinguisher to the VRF using the following format: ip-address:number. The IP address is specified by the BGP router-id statement and the number (which is derived as an unused index in the 0 to 65535 range) is unique across the VRFs.
Note |
In a DCI deployment, for route re-originate with stitching-rt for a particular VRF, using the same Route Distinguisher (RD) between edge DCI gateway and MPLS-VPN PE or same RD between edge DCI gateway and Leaf (ToR) is not supported. |
Enable BGP Routing
Perform this task to enable BGP routing and establish a BGP routing process. Configuring BGP neighbors is included as part of enabling BGP routing.
Note |
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Before you begin
BGP must be able to obtain a router identifier (for example, a configured loopback address). At least, one address family must be configured in the BGP router configuration and the same address family must also be configured under the neighbor.
Note |
If the neighbor is configured as an external BGP (eBGP) peer, you must configure an inbound and outbound route policy on the neighbor using the route-policy command. |
Note |
Instead of configuring an inbound and outbound route policy, you can configure the unsafe eBGP policy to allow all eBGP neighbors to pass routes using the bgp unsafe-ebgp-policy command. |
Procedure
|
Step 1 |
configure Example:Enters mode. |
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Step 2 |
route-policy route-policy-name Example:(Optional) Creates a route policy and enters route policy configuration mode, where you can define the route policy. |
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Step 3 |
end-policy Example:(Optional) Ends the definition of a route policy and exits route policy configuration mode. |
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Step 4 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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Step 5 |
configure Example:Enters mode. |
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Step 6 |
router bgp as-number Example:Specifies the BGP AS number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
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Step 7 |
bgp router-id ip-address Example:Configures the local router with a specified router ID. |
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Step 8 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either the IPv4 or IPv6 address family and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
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Step 9 |
exit Example:Exits the current configuration mode. |
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Step 10 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
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Step 11 |
remote-as as-number Example:Creates a neighbor and assigns a remote autonomous system number to it. |
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Step 12 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either the IPv4 or IPv6 address family and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
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Step 13 |
route-policy route-policy-name { in | out } Example:(Optional) Applies the specified policy to inbound IPv4 unicast routes. |
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Step 14 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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Enabling BGP: Example
The following shows how to enable BGP.
prefix-set static
2020::/64,
2012::/64,
10.10.0.0/16,
10.2.0.0/24
end-set
route-policy pass-all
pass
end-policy
route-policy set_next_hop_agg_v4
set next-hop 10.0.0.1
end-policy
route-policy set_next_hop_static_v4
if (destination in static) then
set next-hop 10.1.0.1
else
drop
endif
end-policy
route-policy set_next_hop_agg_v6
set next-hop 2003::121
end-policy
route-policy set_next_hop_static_v6
if (destination in static) then
set next-hop 2011::121
else
drop
endif
end-policy
router bgp 65000
bgp fast-external-fallover disable
bgp confederation peers
65001
65002
bgp confederation identifier 1
bgp router-id 1.1.1.1
address-family ipv4 unicast
aggregate-address 10.2.0.0/24 route-policy set_next_hop_agg_v4
aggregate-address 10.3.0.0/24
redistribute static route-policy set_next_hop_static_v4
address-family ipv6 unicast
aggregate-address 2012::/64 route-policy set_next_hop_agg_v6
aggregate-address 2013::/64
redistribute static route-policy set_next_hop_static_v6
neighbor 10.0.101.60
remote-as 65000
address-family ipv4 unicast
neighbor 10.0.101.61
remote-as 65000
address-family ipv4 unicast
neighbor 10.0.101.62
remote-as 3
address-family ipv4 unicast
route-policy pass-all in
route-policy pass-all out
neighbor 10.0.101.64
remote-as 5
update-source Loopback0
address-family ipv4 unicast
route-policy pass-all in
route-policy pass-all out
Adjust BGP Timers
BGP uses certain timers to control periodic activities, such as the sending of keepalive messages and the interval after which a neighbor is assumed to be down if no messages are received from the neighbor during the interval. The values set using the timers bgp command in router configuration mode can be overridden on particular neighbors using the timers command in the neighbor configuration mode.
Perform this task to set the timers for BGP neighbors.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
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Step 3 |
timers bgp keepalive hold-time Example:Sets a default keepalive time and a default hold time for all neighbors. |
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Step 4 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
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Step 5 |
timers keepalive hold-time Example:(Optional) Sets the keepalive timer and the hold-time timer for the BGP neighbor. |
|
Step 6 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Change BGP Default Local Preference Value
Perform this task to set the default local preference value for BGP paths.
Procedure
|
Step 1 |
configure Example:Enters mode. |
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Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
bgp default local-preference value Example:Sets the default local preference value from the default of 100, making it either a more preferable path (over 100) or less preferable path (under 100). |
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Step 4 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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Configure MED Metric for BGP
Perform this task to set the multi exit discriminator (MED) to advertise to peers for routes that do not already have a metric set (routes that were received with no MED attribute).
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
default-metric value Example:Sets the default metric, which is used to set the MED to advertise to peers for routes that do not already have a metric set (routes that were received with no MED attribute). |
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Step 4 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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Configure BGP Weights
A weight is a number that you can assign to a path so that you can control the best-path selection process. If you have particular neighbors that you want to prefer for most of your traffic, you can use the weight command to assign a higher weight to all routes learned from that neighbor. Perform this task to assign a weight to routes received from a neighbor.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
|
Step 4 |
remote-as as-number Example:Creates a neighbor and assigns a remote autonomous system number to it. |
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Step 5 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either the IPv4 or IPv6 address family and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
|
Step 6 |
weight weight-value Example:Assigns a weight to all routes learned through the neighbor. |
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Step 7 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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What to do next
You the clear bgp command for the newly configured weight to take effect.
Tune BGP Best-Path Calculation
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Step 1—Compare two paths to determine which is better.
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Step 2—Iterate over all paths and determines which order to compare the paths to select the overall best path.
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Step 3—Determine whether the old and new best paths differ enough so that the new best path should be used.
Note |
The order of comparison determined by Step 2 is important because the comparison operation is not transitive; that is, if three paths, A, B, and C exist, such that when A and B are compared, A is better, and when B and C are compared, B is better, it is not necessarily the case that when A and C are compared, A is better. This nontransitivity arises because the multi exit discriminator (MED) is compared only among paths from the same neighboring autonomous system (AS) and not among all paths. BGP Best Path Algorithm provides additional conceptual details. |
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
bgp bestpath med missing-as-worst Example:Directs the BGP software to consider a missing MED attribute in a path as having a value of infinity, making this path the least desirable path. |
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Step 4 |
bgp bestpath med always Example:Configures the BGP speaker in the specified autonomous system to compare MEDs among all the paths for the prefix, regardless of the autonomous system from which the paths are received. |
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Step 5 |
bgp bestpath med confed Example:Enables BGP software to compare MED values for paths learned from confederation peers. |
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Step 6 |
bgp bestpath as-path ignore Example:Configures the BGP software to ignore the autonomous system length when performing best-path selection. |
|
Step 7 |
bgp bestpath compare-routerid Example:Configure the BGP speaker in the autonomous system to compare the router IDs of similar paths. |
|
Step 8 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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Set BGP Administrative Distance
|
Distance |
Default Value |
Function |
|---|---|---|
|
External |
20 |
Applied to routes learned from eBGP. |
|
Internal |
200 |
Applied to routes learned from iBGP. |
|
Local |
200 |
Applied to routes originated by the router. |
Note |
Distance does not influence the BGP path selection algorithm, but it does influence whether BGP-learned routes are installed in the IP routing table. |
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
|
Step 4 |
distance bgp external-distance internal-distance local-distance Example:Sets the external, internal, and local administrative distances to prefer one class of routes over another. The higher the value, the lower the trust rating. |
|
Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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Indicate BGP Back-door Routes
In most cases, when a route is learned through eBGP, it is installed in the IP routing table because of its distance. Sometimes, however, two ASs have an IGP-learned back-door route and an eBGP-learned route. Their policy might be to use the IGP-learned path as the preferred path and to use the eBGP-learned path when the IGP path is down.
Perform this task to set the administrative distance on an external Border Gateway Protocol (eBGP) route to that of a locally sourced BGP route, causing it to be less preferred than an Interior Gateway Protocol (IGP) route.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either the IPv4 or IPv6 address family and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
|
Step 4 |
network { ip-address / prefix-length | ip-address mask } backdoor Example:Configures the local router to originate and advertise the specified network. |
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Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Back Door: Example
Here, Routers A and C and Routers B and C are running eBGP. Routers A and B are running an IGP (such as Routing Information Protocol [RIP], Interior Gateway Routing Protocol [IGRP], Enhanced IGRP, or Open Shortest Path First [OSPF]). The default distances for RIP, IGRP, Enhanced IGRP, and OSPF are 120, 100, 90, and 110, respectively. All these distances are higher than the default distance of eBGP, which is 20. Usually, the route with the lowest distance is preferred.
Router A receives updates about 160.10.0.0 from two routing protocols: eBGP and IGP. Because the default distance for eBGP is lower than the default distance of the IGP, Router A chooses the eBGP-learned route from Router C. If you want Router A to learn about 160.10.0.0 from Router B (IGP), establish a BGP back door. See .
In the following example, a network back-door is configured:
RP/0/RP0/CPU0:router(config)# router bgp 100
RP/0/RP0/CPU0:router(config-bgp)# address-family ipv4 unicast
RP/0/RP0/CPU0:router(config-bgp-af)# network 160.10.0.0/16 backdoor
Router A treats the eBGP-learned route as local and installs it in the IP routing table with a distance of 200. The network is also learned through Enhanced IGRP (with a distance of 90), so the Enhanced IGRP route is successfully installed in the IP routing table and is used to forward traffic. If the Enhanced IGRP-learned route goes down, the eBGP-learned route is installed in the IP routing table and is used to forward traffic.
Although BGP treats network 160.10.0.0 as a local entry, it does not advertise network 160.10.0.0 as it normally would advertise a local entry.
Configure Aggregate Addresses
Perform this task to create aggregate entries in a BGP routing table.
Note |
For optimal CPU utilization when deploying BGP aggregate for supernet addresses with a higher scale such as internet bgp table, it is recommended to:
|
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either the IPv4 or IPv6 address family and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
|
Step 4 |
aggregate-address address/mask-length [ as-set ] [ as-confed-set ] [ summary-only ] [ route-policy route-policy-name ] Example:Creates an aggregate address. The path advertised for this route is an autonomous system set consisting of all elements contained in all paths that are being summarized.
|
|
Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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Understanding BGP MD5 Authentication
BGP provides a mechanism, known as Message Digest 5 (MD5) authentication, for authenticating a TCP segment between two BGP peers by using a clear text or encrypted password.
MD5 authentication is configured at the BGP neighbor level. BGP peers using MD5 authentication are configured with the same password. If the password authentication fails, then the packets are not transmitted along the segment.
Configuring BGP MD5 Authentication
You can use the configuration in this section to configure BGP MD5 authentication between two BGP peers.
Note |
The configuration for MD5 authentication is identical on both peers. |
Configuration
Use the following configuration to configure BGP MD5:
RP/0/RP0/CPU0:router(config)# router bgp 50
RP/0/RP0/CPU0:router(config-bgp)# address-family ipv4 unicast
RP/0/RP0/CPU0:router(config-bgp-af)# exit
RP/0/RP0/CPU0:router(config-bgp)# neighbor 10.1.1.1
RP/0/RP0/CPU0:router(config-bgp-nbr)# remote-as 51
RP/0/RP0/CPU0:router(config-bgp-nbr)# password encrypted a1b2c3
RP/0/RP0/CPU0:router(config-bgp-nbr)# commit
Running Configuration
Validate the configuration.
RP/0/RP0/CPU0:router# show running-config
...
!
router bgp 50
address-family ipv4 unicast
!
neighbor 10.1.1.1
remote-as 51
password encrypted a1b2c3
!
!
Hiding the Local AS Number for BGP Networks
Changing the autonomous system number is necessary when two separate BGP networks are combined under a single autonomous system. The neighbor local-as command is used to configure BGP peers to support two local autonomous system numbers to maintain peering between two separate BGP networks.
However, when the neighbor local-as command is configured on a BGP peer, the local AS number is automatically prepended to all routes that are learned from eBGP peers by default. This behavior, however, makes changing the autonomous system number for a service provider or large BGP network difficult, because the routes with the prepended AS number are rejected by internal BGP (iBGP) peers that belong to the same AS.
Hiding the local AS number by using the no-prepend command simplifies the process of changing the autonomous system number in a Border Gateway Protocol (BGP) network. Without this feature, internal BGP (iBGP) peers reject external routes from peers with a local AS number in the as-path attribute to prevent routing loops. Hiding the local AS number allows you to transparently change the autonomous system number for the entire BGP network and ensure that routes can be propagated throughout the autonomous system, while the AS number transition is incomplete.
Configuring BGP to Hide the Local AS Number
Hiding the local AS number for eBGP peers by using the no-prepend command can be used to transparently change the AS number of a BGP network, and ensure that routes are propagated throughout the AS during the transition. Because the local AS number is not prepended to these routes, external routes are not rejected by internal peers during the transition from one AS number to another.
This section describes the configuration and verification of the feature.
Note |
BGP prepends the autonomous system number from each BGP network that a route traverses. This behavior is designed to maintain network reachability information and to prevent routing loops from occurring. Configuring the no-prepend command incorrectly could create routing loops. So, the configuration of this command should only be attempted by an experienced network operator. |
Configuration
Use the following configuration to hide the local AS number for eBGP peers.
Router# config
Router(config)# router bgp 100
Router(config-bgp)# address-family ipv4 unicast
Router(config-bgp-af)# network 172.20.1.1 255.255.240.0
Router(config-bgp-af)# neighbor 172.20.1.1
Router(config-bgp-af)# remote-as 150
Router(config-bgp-af)# local-as 300 no-prepend
Router(config-bgp-af)# commit
Running Configuration
RP/0/RP0/CPU0:router# show running-configuration
...
!
router bgp 100
address-family ipv4 unicast
network 10.1.1.1 255.255.0.0
neighbor 10.1.1.1 remote-as 100
neighbor 10.1.1.1 local-as 300 no-prepend
!
Verification
Use the following command to verify your configuration.
RP/0/RP0/CPU0:router# show ip bgp neighbors
BGP neighbor is 10.1.1.1, remote AS 100, local AS 300 no-prepend, external link
BGP version 4, remote router ID 10.1.1.1
BGP state = Established, up for 00:00:49
Last read 00:00:49, hold time is 180, keepalive interval is 60 seconds
Neighbor capabilities:
Route refresh: advertised and received(new)
Address family IPv4 Unicast: advertised and received
IPv4 MPLS Label capability:
Received 10 messages, 1 notifications, 0 in queue
Sent 10 messages, 0 notifications, 0 in queue
Default minimum time between advertisement runs is 30 seconds
Autonomous System Number Formats in BGP
Autonomous system numbers (ASNs) are globally unique identifiers used to identify autonomous systems (ASs) and enable ASs to exchange exterior routing information between neighboring ASs. A unique ASN is allocated to each AS for use in BGP routing. ASNs are encoded as 2-byte numbers and 4-byte numbers in BGP.
RP/0/RP0/CPU0:router(config)# as-format [asdot | asplain]
RP/0/RP0/CPU0:router(config)# as-format asdot
Note |
ASN change for BGP process is not currently supported via commit replace command. |
2-byte Autonomous System Number Format
The 2-byte ASNs are represented in asplain notation. The 2-byte range is 1 to 65535.
4-byte Autonomous System Number Format
To prepare for the eventual exhaustion of 2-byte Autonomous System Numbers (ASNs), BGP has the capability to support 4-byte ASNs. The 4-byte ASNs are represented both in asplain and asdot notations.
The byte range for 4-byte ASNs in asplain notation is 1-4294967295. The AS is represented as a 4-byte decimal number. The 4-byte ASN asplain representation is defined in draft-ietf-idr-as-representation-01.txt.
For 4-byte ASNs in asdot format, the 4-byte range is 1.0 to 65535.65535 and the format is:
high-order-16-bit-value-in-decimal . low-order-16-bit-value-in-decimal
The BGP 4-byte ASN capability is used to propagate 4-byte-based AS path information across BGP speakers that do not support 4-byte AS numbers. See draft-ietf-idr-as4bytes-12.txt for information on increasing the size of an ASN from 2 bytes to 4 bytes. AS is represented as a 4-byte decimal number
as-format Command
The as-format command configures the ASN notation to asdot. The default value, if the as-format command is not configured, is asplain.
BGP Multi-Instance and Multi-AS
-
Mechanism to consolidate the services provided by multiple routers using a common routing infrastructure into a single IOS-XR router.
-
Mechanism to achieve AF isolation by configuring the different AFs in different BGP instances.
-
Means to achieve higher session scale by distributing the overall peering sessions between multiple instances.
-
Mechanism to achieve higher prefix scale (especially on a RR) by having different instances carrying different BGP tables.
-
Improved BGP convergence under certain scenarios.
-
All BGP functionalities including NSR are supported for all the instances.
-
The load and commit router-level operations can be performed on previously verified or applied configurations.
Restrictions
-
The router supports maximum of 4 BGP instances.
-
Each BGP instance needs a unique router-id.
-
Only one Address Family can be configured under each BGP instance (VPNv4, VPNv6 and RT-Constrain can be configured under multiple BGP instances).
-
IPv4/IPv6 Unicast should be within the same BGP instance in which IPv4/IPv6 Labeled-Unicast is configured.
-
IPv4/IPv6 Multicast should be within the same BGP instance in which IPv4/IPv6 Unicast is configured.
-
All configuration changes for a single BGP instance can be committed together. However, configuration changes for multiple instances cannot be committed together.
-
Cisco recommends that BGP update-source should be unique in the default VRF over all instances while peering with the same remote router.
Configure Multiple BGP Instances for a Specific Autonomous System
Perform this task to configure multiple BGP instances for a specific autonomous system. All configuration changes for a single BGP instance can be committed together. However, configuration changes for multiple instances cannot be committed together.
Procedure
|
Step 1 |
configure Example:Enters mode. |
||
|
Step 2 |
router bgp as-number [instance instance name ] Example:Enters BGP configuration mode for the user specified BGP instance. |
||
|
Step 3 |
bgp router-id ip-address Example:Configures a fixed router ID for the BGP-speaking router (BGP instance).
|
||
|
Step 4 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
BGP Routing Domain Confederation
One way to reduce the iBGP mesh is to divide an autonomous system into multiple sub-autonomous systems and group them into a single confederation. To the outside world, the confederation looks like a single autonomous system. Each autonomous system is fully meshed within itself and has a few connections to other autonomous systems in the same confederation. Although the peers in different autonomous systems have eBGP sessions, they exchange routing information as if they were iBGP peers. Specifically, the next hop, MED, and local preference information is preserved. This feature allows you to retain a single IGP for all of the autonomous systems.
Configure Routing Domain Confederation for BGP
Perform this task to configure the routing domain confederation for BGP. This includes specifying a confederation identifier and autonomous systems that belong to the confederation.
Configuring a routing domain confederation reduces the internal BGP (iBGP) mesh by dividing an autonomous system into multiple autonomous systems and grouping them into a single confederation. Each autonomous system is fully meshed within itself and has a few connections to another autonomous system in the same confederation. The confederation maintains the next hop and local preference information, and that allows you to retain a single Interior Gateway Protocol (IGP) for all autonomous systems. To the outside world, the confederation looks like a single autonomous system.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
bgp confederation identifier as-number Example:Specifies a BGP confederation identifier. |
|
Step 4 |
bgp confederation peers as-number Example:Specifies that the BGP autonomous systems belong to a specified BGP confederation identifier. You can associate multiple AS numbers to the same confederation identifier, as shown in the example. |
|
Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
BGP Confederation: Example
The following is a sample configuration that shows several peers in a confederation. The confederation consists of three internal autonomous systems with autonomous system numbers 6001, 6002, and 6003. To the BGP speakers outside the confederation, the confederation looks like a normal autonomous system with autonomous system number 666 (specified using the bgp confederation identifier command).
In a BGP speaker in autonomous system 6001, the bgp confederation peers command marks the peers from autonomous systems 6002 and 6003 as special eBGP peers. Hence, peers 171.16 .232.55 and 171.16 .232.56 get the local preference, next hop, and MED unmodified in the updates. The router at 171 .19 .69.1 is a normal eBGP speaker, and the updates received by it from this peer are just like a normal eBGP update from a peer in autonomous system 666.
router bgp 6001
bgp confederation identifier 666
bgp confederation peers
6002
6003
exit
address-family ipv4 unicast
neighbor 171.16.232.55
remote-as 6002
exit
address-family ipv4 unicast
neighbor 171.16.232.56
remote-as 6003
exit
address-family ipv4 unicast
neighbor 171.19.69.1
remote-as 777
router bgp 6002
bgp confederation identifier 666
bgp confederation peers
6001
6003
exit
address-family ipv4 unicast
neighbor 171.17.70.1
remote-as 6002
exit
address-family ipv4 unicast
neighbor 171.19.232.57
remote-as 6001
exit
address-family ipv4 unicast
neighbor 171.19.232.56
remote-as 6003
exit
address-family ipv4 unicast
neighbor 171.19.99.2
remote-as 700
exit
address-family ipv4 unicast
route-policy pass-all in
route-policy pass-all out
router bgp 6003
bgp confederation identifier 666
bgp confederation peers
6001
6002
exit
address-family ipv4 unicast
neighbor 171.19.232.57
remote-as 6001
exit
address-family ipv4 unicast
neighbor 171.19.232.55
remote-as 6002
exit
address-family ipv4 unicast
neighbor 192.168.200.200
remote-as 701
exit
address-family ipv4 unicast
route-policy pass-all in
route-policy pass-all out
router bgp 701
address-family ipv4 unicast
neighbor 172.16.232.56
remote-as 666
exit
address-family ipv4 unicast
route-policy pass-all in
route-policy pass-all out
exit
address-family ipv4 unicast
neighbor 192.168.200.205
remote-as 701
BGP Additional Paths
The Border Gateway Protocol (BGP) Additional Paths feature modifies the BGP protocol machinery for a BGP speaker to be able to send multiple paths for a prefix. This gives 'path diversity' in the network. The add path enables BGP prefix independent convergence (PIC) at the edge routers.
BGP add path enables add path advertisement in an iBGP network and advertises the following types of paths for a prefix:
-
Backup paths—to enable fast convergence and connectivity restoration.
-
Group-best paths—to resolve route oscillation.
-
All paths—to emulate an iBGP full-mesh.
Configure BGP Additional Paths
Perform these tasks to configure BGP Additional Paths capability:
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
route-policy route-policy-name Example:Defines the route policy and enters route-policy configuration mode. |
|
Step 3 |
if conditional-expression then action-statement else Example:Decides the actions and dispositions for the given route. |
|
Step 4 |
pass endif Example:Passes the route for processing and ends the if statement. |
|
Step 5 |
end-policy Example:Ends the route policy definition of the route policy and exits route-policy configuration mode. |
|
Step 6 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 7 |
address-family {ipv4 {unicast } | ipv6 {unicast | l2vpn vpls-vpws | vpnv4 unicast | vpnv6 unicast } Example:Specifies the address family and enters address family configuration submode. |
|
Step 8 |
additional-paths receive Example:Configures receive capability of multiple paths for a prefix to the capable peers. |
|
Step 9 |
additional-paths send Example:Configures send capability of multiple paths for a prefix to the capable peers . |
|
Step 10 |
additional-paths selection route-policy route-policy-name Example:Configures additional paths selection capability for a prefix. |
|
Step 11 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
BGP Maximum Prefix
The maximum-prefix feature imposes a maximum limit on the number of prefixes that are received from a neighbor for a given address family. Whenever the number of prefixes received exceeds the maximum number configured, the BGP session is terminated, which is the default behavior, after sending a cease notification to the neighbor. The session is down until a manual clear is performed by the user. The session can be resumed by using the clear bgp command. It is possible to configure a period after which the session can be automatically brought up by using the maximum-prefix command with the restart keyword. The maximum prefix limit can be configured by the user. Default limits are used if the user does not configure the maximum number of prefixes for the address family.
Discard Extra Paths
An option to discard extra paths is added to the maximum-prefix configuration. Configuring the discard extra paths option drops all excess prefixes received from the neighbor when the prefixes exceed the configured maximum value. This drop does not, however, result in session flap.
The benefits of discard extra paths option are:
-
Limits the memory footstamp of BGP.
-
Stops the flapping of the peer if the paths exceed the set limit.
When the discard extra paths configuration is removed, BGP sends a route-refresh message to the neighbor if it supports the refresh capability; otherwise the session is flapped.
On the same lines, the following describes the actions when the maximum prefix value is changed:
-
If the maximum value alone is changed, a route-refresh message is sourced, if applicable.
-
If the new maximum value is greater than the current prefix count state, the new prefix states are saved.
-
If the new maximum value is less than the current prefix count state, then some existing prefixes are deleted to match the new configured state value.
There is currently no way to control which prefixes are deleted.
Configure Discard Extra Paths
The discard extra paths option in the maximum-prefix configuration allows you to drop all excess prefixes received from the neighbor when the prefixes exceed the configured maximum value. This drop does not, however, result in session flap.
The benefits of discard extra paths option are:
-
Limits the memory footstamp of BGP.
-
Stops the flapping of the peer if the paths exceed the set limit.
Note |
|
Perform this task to configure BGP maximum-prefix discard extra paths.
Procedure
|
Step 1 |
configure Example:Enters XR Config mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
|
Step 4 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either the IPv4 or IPv6 address family and enters address family configuration submode. |
|
Step 5 |
maximum-prefix maximum discard-extra-paths Example:Configures a limit to the number of prefixes allowed. Configures discard extra paths to discard extra paths when the maximum prefix limit is exceeded. |
|
Step 6 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Example
The following example shows how to configure discard extra paths feature for the IPv4 address family:
RP/0//CPU0:router# configure
RP/0//CPU0:router(config)# router bgp 10
RP/0//CPU0:router(config-bgp)# neighbor 10.0.0.1
RP/0//CPU0:router(config-bgp-nbr)# address-family ipv4 unicast
RP/0//CPU0:router(config-bgp-nbr-af)# maximum-prefix 1000 discard-extra-paths
RP/0//CPU0:router(config-bgp-vrf-af)# commit
The show bgp neighbor output shows the cumulative number for the Prefix advertised count if the same prefixes are withdrawn and re-advertised.
The following screen output shows details about the discard extra paths option:
RP/0//CPU0:ios# show bgp neighbor 10.0.0.1
BGP neighbor is 10.0.0.1
Remote AS 10, local AS 10, internal link
Remote router ID 0.0.0.0
BGP state = Idle (No best local address found)
Last read 00:00:00, Last read before reset 00:00:00
Hold time is 180, keepalive interval is 60 seconds
Configured hold time: 180, keepalive: 60, min acceptable hold time: 3
Last write 00:00:00, attempted 0, written 0
Second last write 00:00:00, attempted 0, written 0
Last write before reset 00:00:00, attempted 0, written 0
Second last write before reset 00:00:00, attempted 0, written 0
Last write pulse rcvd not set last full not set pulse count 0
Last write pulse rcvd before reset 00:00:00
Socket not armed for io, not armed for read, not armed for write
Last write thread event before reset 00:00:00, second last 00:00:00
Last KA expiry before reset 00:00:00, second last 00:00:00
Last KA error before reset 00:00:00, KA not sent 00:00:00
Last KA start before reset 00:00:00, second last 00:00:00
Precedence: internet
Multi-protocol capability not received
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Minimum time between advertisement runs is 0 secs
For Address Family: IPv4 Unicast
BGP neighbor version 0
Update group: 0.1 Filter-group: 0.0 No Refresh request being processed
Route refresh request: received 0, sent 0
0 accepted prefixes, 0 are bestpaths
Cumulative no. of prefixes denied: 0.
Prefix advertised 0, suppressed 0, withdrawn 0
Maximum prefixes allowed 10 (discard-extra-paths) <<<<<<<<<<<<<<<<<<<<<
Threshold for warning message 75%, restart interval 0 min
AIGP is enabled
An EoR was not received during read-only mode
Last ack version 1, Last synced ack version 0
Outstanding version objects: current 0, max 0
Additional-paths operation: None
Send Multicast Attributes
Connections established 0; dropped 0
Local host: 0.0.0.0, Local port: 0, IF Handle: 0x00000000
Foreign host: 10.0.0.1, Foreign port: 0
Last reset 00:00:00
BGP Best-External Path
The best–external path functionality supports advertisement of the best–external path to the iBGP and Route Reflector peers when a locally selected bestpath is from an internal peer. BGP selects one best path and one backup path to every destination. By default, selects one best path . Additionally, BGP selects another bestpath from among the remaining external paths for a prefix. Only a single path is chosen as the best–external path and is sent to other PEs as the backup path. BGP calculates the best–external path only when the best path is an iBGP path. If the best path is an eBGP path, then best–external path calculation is not required.
The procedure to determine the best–external path is as follows:
-
Determine the best path from the entire set of paths available for a prefix.
-
Eliminate the current best path.
-
Eliminate all the internal paths for the prefix.
-
From the remaining paths, eliminate all the paths that have the same next hop as that of the current best path.
-
Rerun the best path algorithm on the remaining set of paths to determine the best–external path.
BGP considers the external and confederations BGP paths for a prefix to calculate the best–external path. BGP advertises the best path and the best–external path as follows:
-
On the primary PE—advertises the best path for a prefix to both its internal and external peers
-
On the backup PE—advertises the best path selected for a prefix to the external peers and advertises the best–external path selected for that prefix to the internal peers
Configure Best-External Path Advertisement
Perform the following tasks to advertise the best–external path to the iBGP and route-reflector peers:
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
Do one of the following
Example:Specifies the address family or VRF address family and enters the address family or VRF address family configuration submode. |
|
Step 4 |
advertise best-external Example:Advertise the best–external path to the iBGP and route-reflector peers. |
|
Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
BGP Local Label Retention
When a primary PE-CE link fails, BGP withdraws the route corresponding to the primary path along with its local label and programs the backup path in the Routing Information Base (RIB) and the Forwarding Information Base (FIB), by default.
However, until all the internal peers of the primary PE reconverge to use the backup path as the new bestpath, the traffic continues to be forwarded to the primary PE with the local label that was allocated for the primary path. Hence the previously allocated local label for the primary path must be retained on the primary PE for some configurable time after the reconvergence. BGP Local Label Retention feature enables the retention of the local label for a specified period. If no time is specified, the local lable is retained for a default value of five minutes.
Retain Allocated Local Label for Primary Path
Perform the following tasks to retain the previously allocated local label for the primary path on the primary PE for some configurable time after reconvergence:
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
address-family { vpnv4 unicast | vpnv6 unicast } Example:Specifies the address family and enters the address family configuration submode. |
|
Step 4 |
retain local-label minutes Example:Retains the previously allocated local label for the primary path on the primary PE for 10 minutes after reconvergence. |
|
Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Allocated Local Label Retention: Example
The following example shows how to retain the previously allocated local label for the primary path on the primary PE for 10 minutes after reconvergence:
router bgp 100
address-family l2vpn vpnv4 unicast
retain local-label 10
end
iBGP Multipath Load Sharing
When a Border Gateway Protocol (BGP) speaking router that has no local policy configured, receives multiple network layer reachability information (NLRI) from the internal BGP (iBGP) for the same destination, the router will choose one iBGP path as the best path. The best path is then installed in the IP routing table of the router. The iBGP Multipath Load Sharing feature enables the BGP speaking router to select multiple iBGP paths as the best paths to a destination. The best paths or multipaths are then installed in the IP routing table of the router.
iBGP Multipath Load Sharing Reference provides additional details.
Configure iBGP Multipath Load Sharing
Perform this task to configure the iBGP Multipath Load Sharing:
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
address-family {ipv4 |ipv6 } {unicast |multicast } Example:Specifies either the IPv4 or IPv6 address family and enters address family configuration submode. |
|
Step 4 |
maximum-paths ibgp number Example:Configures the maximum number of iBGP paths for load sharing. |
|
Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
iBGP Multipath Loadsharing Configuration: Example
The following is a sample configuration where 30 paths are used for loadsharing:
router bgp 100
address-family ipv4 multicast
maximum-paths ibgp 30
!
!
end
Persistent Loadbalancing
Traditional ECMP or equal cost multipath loadbalances traffic over a number of available paths towards a destination. When one path fails, the traffic gets re-shuffled over the available number of paths. This flow distribution can be a problem in data center loadbalancing.
Persistent Loadbalancing or Sticky ECMP defines a prefix in such a way that it do not rehash flows on existing paths and only replace those bucket assignments of the failed server. The advantage is that the established sessions to servers will not get rehashed.
The following section describes how you can configure persistent load balancing:
/*Configure persistent load balancing. */
Router(config)# router bgp 7500
Router(config-bgp)# address-family ipv4 unicast
Router(config-bgp-af)# table-policy sticky-ecmp
Router(config-bgp-af)# bgp attribute-download
Router(config-bgp-af)# maximum-paths ebgp 64
Router(config-bgp-af)# maximum-paths ibgp 32
Router(config-bgp-af)# exit
Router(config-bgp)# exit
Router(config)# route-policy sticky-ecmp
Router(config-rpl)# if destination in (192.1.1.1/24) then
Router(config-rpl-if)# set load-balance ecmp-consistent
Router(config-rpl-if)# else
Router(config-rpl-else)# pass
Router(config-rpl-else)# endif
RP/0/0/CPU0:ios(config-rpl)# end-policy
RP/0/0/CPU0:ios(config)#
/* Enable autocovery and hence recover the original hashing state
after failed paths become active. */
Router(config)# cef consistent-hashing auto-recovery
/* Recover to the original hashing state after failed paths come up
and avoid affecting newly formed flows after path failure. */
Router(config)# clear route 192.0.2.0/24
Running Configuration
/* Configure persistent loadbalancing. */
router bgp 7500
address-family ipv4 unicast
table-policy sticky-ecmp
bgp attribute-download
maximum-paths ebgp 64
maximum-paths ibgp 32
cef consistent-hashing auto-recovery
clear route 192.0.2.0/24
Verification
Verify that the path distribution with persistent loadbalancing is configured.
The following show output displays the status of path distribution before a link fails. In this output, three paths are identified with three next hops (10.1/2/3.0.1) through three different GigabitEthernet interfaces.
show cef 192.0.2.0/24
LDI Update time Sep 5 11:22:38.201
via 10.1.0.1/32, 3 dependencies, recursive, bgp-multipath [flags 0x6080]
path-idx 0 NHID 0x0 [0x57ac4e74 0x0]
next hop 10.1.0.1/32 via 10.1.0.1/32
via 10.2.0.1/32, 3 dependencies, recursive, bgp-multipath [flags 0x6080]
path-idx 1 NHID 0x0 [0x57ac4a74 0x0]
next hop 10.2.0.1/32 via 10.2.0.1/32
via 10.3.0.1/32, 3 dependencies, recursive, bgp-multipath [flags 0x6080]
path-idx 2 NHID 0x0 [0x57ac4f74 0x0]
next hop 10.3.0.1/32 via 10.3.0.1/32
Load distribution (consistent): 0 1 2 (refcount 1)
Hash OK Interface Address
0 Y GigabitEthernet0/0/0/0 10.1.0.1
1 Y GigabitEthernet0/0/0/1 10.2.0.1
2 Y GigabitEthernet0/0/0/2 10.3.0.1
The following show output displays the status of the path distribution after a link fails. The replacement of bucket 1 with GigabitEthernet 0/0/0/0 and the "*" symbol denotes that this path is a replacement for a failed path.
show cef 192.0.2.0/24
LDI Update time Sep 5 11:23:13.434
via 10.1.0.1/32, 3 dependencies, recursive, bgp-multipath [flags 0x6080]
path-idx 0 NHID 0x0 [0x57ac4e74 0x0]
next hop 10.1.0.1/32 via 10.1.0.1/32
via 10.3.0.1/32, 3 dependencies, recursive, bgp-multipath [flags 0x6080]
path-idx 1 NHID 0x0 [0x57ac4f74 0x0]
next hop 10.3.0.1/32 via 10.3.0.1/32
Load distribution (consistent) : 0 1 2 (refcount 1)
Hash OK Interface Address
0 Y GigabitEthernet0/0/0/0 10.1.0.1
1* Y GigabitEthernet0/0/0/0 10.1.0.1
2 Y GigabitEthernet0/0/0/2 10.3.0.1 Route Dampening
Route dampening is a BGP feature that minimizes the propagation of flapping routes across an internetwork. A route is considered to be flapping when it is repeatedly available, then unavailable, then available, then unavailable, and so on.
For example, consider a network with three BGP autonomous systems: autonomous system 1, autonomous system 2, and autonomous system 3. Suppose the route to network A in autonomous system 1 flaps (it becomes unavailable). Under circumstances without route dampening, the eBGP neighbor of autonomous system 1 to autonomous system 2 sends a withdraw message to autonomous system 2. The border router in autonomous system 2, in turn, propagates the withdrawal message to autonomous system 3. When the route to network A reappears, autonomous system 1 sends an advertisement message to autonomous system 2, which sends it to autonomous system 3. If the route to network A repeatedly becomes unavailable, then available, many withdrawal and advertisement messages are sent. Route flapping is a problem in an internetwork connected to the Internet, because a route flap in the Internet backbone usually involves many routes.
The route dampening feature minimizes the flapping problem as follows. Suppose again that the route to network A flaps. The router in autonomous system 2 (in which route dampening is enabled) assigns network A a penalty of 1000 and moves it to history state. The router in autonomous system 2 continues to advertise the status of the route to neighbors. The penalties are cumulative. When the route flaps so often that the penalty exceeds a configurable suppression limit, the router stops advertising the route to network A, regardless of how many times it flaps. Thus, the route is dampened.
The penalty placed on network A is decayed until the reuse limit is reached, upon which the route is once again advertised. At half of the reuse limit, the dampening information for the route to network A is removed.
Note |
No penalty is applied to a BGP peer reset when route dampening is enabled, even though the reset withdraws the route. |
Configuring BGP Route Dampening
Perform this task to configure and monitor BGP route dampening.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either the IPv4 or IPv6 address family and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
|
Step 4 |
bgp dampening [ half-life [ reuse suppress max-suppress-time ] | route-policy route-policy-name ] Example:Configures BGP dampening for the specified address family. |
|
Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Routing Policy Enforcement
External BGP (eBGP) neighbors must have an inbound and outbound policy configured. If no policy is configured, no routes are accepted from the neighbor, nor are any routes advertised to it. This added security measure ensures that routes cannot accidentally be accepted or advertised in the case of a configuration omission error.
Note |
This enforcement affects only eBGP neighbors (neighbors in a different autonomous system than this router). For internal BGP (iBGP) neighbors (neighbors in the same autonomous system), all routes are accepted or advertised if there is no policy. |
Apply Policy When Updating Routing Table
The table policy feature in BGP allows you to configure traffic index values on routes as they are installed in the global routing table. This feature is enabled using the table-policy command and supports the BGP policy accounting feature. Table policy also provides the ability to drop routes from the RIB based on match criteria. This feature can be useful in certain applications and should be used with caution as it can easily create a routing traffic drop where BGP advertises routes to neighbors that BGP does not install in its global routing table and forwarding table.
Perform this task to apply a routing policy to routes being installed into the routing table.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either the IPv4 or IPv6 address family and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
|
Step 4 |
table-policy policy-name Example:Applies the specified policy to routes being installed into the routing table. |
|
Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Applying routing policy: Example
In the following example, for an eBGP neighbor, if all routes should be accepted and advertised with no modifications, a simple pass-all policy is configured:
RP/0/RP0/CPU0:router(config)# route-policy pass-all
RP/0/RP0/CPU0:router(config-rpl)# pass
RP/0/RP0/CPU0:router(config-rpl)# end-policy
RP/0/RP0/CPU0:router(config)# commit
Use the route-policy (BGP) command in the neighbor address-family configuration mode to apply the pass-all policy to a neighbor. The following example shows how to allow all IPv4 unicast routes to be received from neighbor 192.168.40.42 and advertise all IPv4 unicast routes back to it:
RP/0/RP0/CPU0:router(config)# router bgp 1
RP/0/RP0/CPU0:router(config-bgp)# neighbor 192.168.40.24
RP/0/RP0/CPU0:router(config-bgp-nbr)# remote-as 21
RP/0/RP0/CPU0:router(config-bgp-nbr)# address-family ipv4 unicast
RP/0/RP0/CPU0:router(config-bgp-nbr-af)# route-policy pass-all in
RP/0/RP0/CPU0:router(config-bgp-nbr-af)# route-policy pass-all out
RP/0/RP0/CPU0:router(config-bgp-nbr-af)# commit
Use the show bgp summary command to display eBGP neighbors that do not have both an inbound and outbound policy for every active address family. In the following example, such eBGP neighbors are indicated in the output with an exclamation (!) mark:
RP/0/RP0/CPU0:router# show bgp all all summary
Address Family: IPv4 Unicast
============================
BGP router identifier 10.0.0.1, local AS number 1
BGP generic scan interval 60 secs
BGP main routing table version 41
BGP scan interval 60 secs
BGP is operating in STANDALONE mode.
Process RecvTblVer bRIB/RIB SendTblVer
Speaker 41 41 41
Neighbor Spk AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down St/PfxRcd
10.0.101.1 0 1 919 925 41 0 0 15:15:08 10
10.0.101.2 0 2 0 0 0 0 0 00:00:00 Idle
Mitigating DDoS Attacks Using Remote Traffic Black Holing (RTBH)
|
Feature Name |
Release Information |
Description |
|---|---|---|
|
Source-based Remote Traffic Black Holing (S-RTBH) |
Release 24.4.1 |
Source-based RTBH (S-RTBH) is a network security technique used to drop traffic originating from specific malicious source IP addresses. It leverages BGP updates and Unicast Reverse Path Forwarding (uRPF) to ensure that unwanted traffic is discarded at the network edge. |
Remote Traffic Black Holing (RTBH) is a network security technique used to mitigate Distributed Denial of Service (DDoS) attacks and other unwanted traffic. It involves discarding malicious or unwanted traffic at the network edge before it can enter the core network. This preemptive measure helps protect network infrastructure and services from potential disruptions and damage caused by such attacks.
Types of Remote Traffic Black Holing (RTBH)
RTBH can be implemented in two primary ways:
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Destination-based RTBH (D-RTBH): This method allows network operators to mitigate DoS attacks by discarding malicious traffic destined for a specific target IP address.
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Source-based RTBH (S-RTBH): This method allows network operators to mitigate attacks by discarding traffic originating from malicious source IP addresses.
How RTBH Works: D-RTBH vs S-RTBH
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Attack Detection: Network monitoring tools, like NetFlow, detect anomalous traffic patterns indicating a potential DoS attack.
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D-RTBH: Targets a specific IP address as the victim.
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S-RTBH: Identifies a specific IP address as the attacker.
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BGP Update Initiation: A network administrator configures a static route on a trigger router, setting the next hop to Null0. The trigger router tags the route with a BGP community indicating it should be blackholed.
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D-RTBH: Static route is configured for the victim’s IP address.
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S-RTBH: Static route is configured for the attacker’s IP address.
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BGP Update Propagation: The trigger router shares the static route with all connected routers using BGP.
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Traffic Dropping:
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D-RTBH: The next hop for the destination IP (victim’s IP) is set to Null0. Traffic to the victim's IP is dropped at the edge routers, preventing it from reaching the core network or the target.
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S-RTBH: Edge routers are configured with Unicast Reverse Path Forwarding (uRPF) on their ingress interfaces to validate the source IP of incoming packets. Traffic from the attacker’s IP is dropped at the edge routers when uRPF determines that the source IP routes to Null0, preventing it from entering the core network or reaching the target.
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Use Cases for Remote Traffic Black Holing: Destination-Based (D-RTBH) vs. Source-Based (S-RTBH)
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D-RTBH: Suitable for scenarios where a specific server or network segment is under attack. For example, if a web server is being targeted by a DDoS attack, D-RTBH can drop all traffic directed to the server's IP address.
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S-RTBH: Suitable for scenarios where the attack originates from a specific source IP address. For instance, if an attacker is flooding the network from a known IP address, S-RTBH can drop all traffic from the attacker’s IP.
Configuring Source-Based RTBH
Before you begin
Procedure
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Step 1 |
Enable uRPF on Ingress Interfaces: Configure uRPF on the ingress interfaces of edge routers to validate the source IP address of incoming packets. |
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Step 2 |
Configure Static Route on Trigger Router: Set up a static route for the attacker’s IP address on a trigger router, setting the next hop to Null0 and tagging it with a BGP community. |
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Step 3 |
Apply Route Policy on Trigger Router: Apply a route policy to tag the static route with a BGP community indicating it should be blackholed. |
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Step 4 |
Redistribute Static Route into BGP: Redistribute the static route into BGP using the configured route policy. |
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Step 5 |
Configure Route Policy on Edge Routers Set up a route policy on edge routers to match the BGP community and set the next hop to discard. |
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Step 6 |
Propagate BGP Update Ensure that the BGP update propagates to all BGP peers, including edge routers. |
Verifying Source-Based RTBH Configuration
Procedure
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Step 1 |
Verify BGP Entry Check the BGP table to ensure that the prefix 10.7.7.7/32 is flagged as Nexthop-discard. |
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Step 2 |
Verify Specific BGP Route Check the specific BGP route for the prefix 10.7.7.7/32 to ensure it is correctly flagged and set to Null0. |
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Step 3 |
Verify Routing Table Entry Check the routing table to ensure that the route for 10.7.7.7/32 is set to Null0. |
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Step 4 |
Verify Drop Statistics on Ingress Linecards Check the drop statistics on the ingress linecards to ensure that Null0 drops are occurring |
Configuring and Verifying Destination-Based Remote Traffic Black Holing (D-RTBH)
The configuration and verification steps for D-RTBH are similar to those for S-RTBH. The key differences are:
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Configure a static route for the victim’s IP address instead of the attacker’s IP address.
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When verifying the static route, BGP redistribution, and traffic dropping, use the victim’s IP address.
Configure BGP Neighbor Group and Neighbors
Perform this task to configure BGP neighbor groups and apply the neighbor group configuration to a neighbor. A neighbor group is a template that holds address family-independent and address family-dependent configurations that are associated with the neighbor.
After a neighbor group is configured, each neighbor can inherit the configuration through the usecommand. If a neighbor is configured to use a neighbor group, the neighbor (by default) inherits the entire configuration of the neighbor group, which includes the address family-independent and address family-dependent configurations. The inherited configuration can be overridden if you directly configure commands for the neighbor or configure session groups or address family groups through the usecommand.
You can configure an address family-independent configuration under the neighbor group. An address family-dependent configuration requires you to configure the address family under the neighbor group to enter address family submode. From neighbor group configuration mode, you can configure address family-independent parameters for the neighbor group. Use the address-familycommand when in the neighbor group configuration mode. After specifying the neighbor group name using the neighbor group command, you can assign options to the neighbor group.
Note |
All commands that can be configured under a specified neighbor group can be configured under a neighbor. |
Note |
In Cisco IOS-XR versions prior to 6.3.2, you cannot remove a autonomous system that belongs to a BGP neighbor and move it under a BGP neigbhorgroup using a single IOS-XR commit. Effective with 6.3.2, you can move the autonoums system from a neighbor to a neighbor group in a single IOS-XR commit. |
Procedure
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Step 1 |
configure Example:Enters mode. |
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Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
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Step 3 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
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Step 4 |
exit Example:Exits the current configuration mode. |
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Step 5 |
neighbor-group name Example:Places the router in neighbor group configuration mode. |
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Step 6 |
remote-as as-number Example:Creates a neighbor and assigns a remote autonomous system number to it. |
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Step 7 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
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Step 8 |
route-policy route-policy-name { in | out } Example:(Optional) Applies the specified policy to inbound IPv4 unicast routes. |
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Step 9 |
exit Example:Exits the current configuration mode. |
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Step 10 |
exit Example:Exits the current configuration mode. |
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Step 11 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
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Step 12 |
use neighbor-group group-name Example:(Optional) Specifies that the BGP neighbor inherit configuration from the specified neighbor group. |
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Step 13 |
remote-as as-number Example:Creates a neighbor and assigns a remote autonomous system number to it. |
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Step 14 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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BGP Neighbor Configuration: Example
The following example shows how BGP neighbors on an autonomous system are configured to share information. In the example, a BGP router is assigned to autonomous system 109, and two networks are listed as originating in the autonomous system. Then the addresses of three remote routers (and their autonomous systems) are listed. The router being configured shares information about networks 172 .16 .0.0 and 192.168 .7.0 with the neighbor routers. The first router listed is in a different autonomous system; the second neighbor and remote-as commands specify an internal neighbor (with the same autonomous system number) at address 172 .26 .234.2; and the third neighbor and remote-as commands specify a neighbor on a different autonomous system.
route-policy pass-all
pass
end-policy
router bgp 109
address-family ipv4 unicast
network 172.16.0.0 255.255.0.0
network 192.168.7.0 255.255.0.0
neighbor 172.16.200.1
remote-as 167
address-family ipv4 unicast
route-policy pass-all in
route-policy pass-out out
neighbor 172.26.234.2
remote-as 109
address-family ipv4 unicast
neighbor 172.26.64.19
remote-as 99
address-family ipv4 unicast
route-policy pass-all in
route-policy pass-all out
Disable BGP Neighbor
Perform this task to administratively shut down a neighbor session without removing the configuration.
Procedure
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Step 1 |
configure Example:Enters mode. |
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Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
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Step 3 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
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Step 4 |
shutdown Example:Disables all active sessions for the specified neighbor. |
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Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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Resetting Neighbors Using BGP Inbound Soft Reset
Perform this task to trigger an inbound soft reset of the specified address families for the specified group or neighbors. The group is specified by the * , ip-address , as-number , or external keywords and arguments.
Resetting neighbors is useful if you change the inbound policy for the neighbors or any other configuration that affects the sending or receiving of routing updates. If an inbound soft reset is triggered, BGP sends a REFRESH request to the neighbor if the neighbor has advertised the ROUTE_REFRESH capability. To determine whether the neighbor has advertised the ROUTE_REFRESH capability, use the show bgp neighbors command.
Procedure
| Command or Action | Purpose | |
|---|---|---|
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Step 1 |
show bgp neighbors Example: |
Verifies that received route refresh capability from the neighbor is enabled. |
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Step 2 |
soft [ in [ prefix-filter ] | out ] Example: |
Soft resets a BGP neighbor.
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Resetting Neighbors Using BGP Outbound Soft Reset
Perform this task to trigger an outbound soft reset of the specified address families for the specified group or neighbors. The group is specified by the * , ip-address , as-number , or external keywords and arguments.
Resetting neighbors is useful if you change the outbound policy for the neighbors or any other configuration that affects the sending or receiving of routing updates.
If an outbound soft reset is triggered, BGP resends all routes for the address family to the given neighbors.
To determine whether the neighbor has advertised the ROUTE_REFRESH capability, use the show bgp neighbors command.
Procedure
| Command or Action | Purpose | |
|---|---|---|
|
Step 1 |
show bgp neighbors Example: |
Verifies that received route refresh capability from the neighbor is enabled. |
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Step 2 |
Example: |
Soft resets a BGP neighbor.
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Reset Neighbors Using BGP Hard Reset
Perform this task to reset neighbors using a hard reset. A hard reset removes the TCP connection to the neighbor, removes all routes received from the neighbor from the BGP table, and then re-establishes the session with the neighbor. If the graceful keyword is specified, the routes from the neighbor are not removed from the BGP table immediately, but are marked as stale. After the session is re-established, any stale route that has not been received again from the neighbor is removed.
Procedure
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clear bgp { ipv4 { unicast | labeled-unicast | all | tunnel tunnel | mdt } | ipv6 unicast | all | labeled-unicast } | all { unicast | multicast | all | labeled-unicast | mdt | tunnel } | vpnv4 unicast | vrf { vrf-name | all } { ipv4 unicast | labeled-unicast } | ipv6 unicast } | vpnv6 unicast } { * | ip-address | as as-number | external } [ graceful ] soft [ in [ prefix-filter ] | out ] clear bgp { ipv4 | ipv6} { unicast | labeled-unicast } Example:Clears a BGP neighbor.
The graceful keyword specifies a graceful restart. |
Configure Software to Store Updates from Neighbor
Perform this task to configure the software to store updates received from a neighbor.
The soft-reconfiguration inbound command causes a route refresh request to be sent to the neighbor if the neighbor is route refresh capable. If the neighbor is not route refresh capable, the neighbor must be reset to relearn received routes using the clear bgp soft command.
Note |
Storing updates from a neighbor works only if either the neighbor is route refresh capable or the soft-reconfiguration inbound command is configured. Even if the neighbor is route refresh capable and the soft-reconfiguration inbound command is configured, the original routes are not stored unless the always option is used with the command. The original routes can be easily retrieved with a route refresh request. Route refresh sends a request to the peer to resend its routing information. The soft-reconfiguration inbound command stores all paths received from the peer in an unmodified form and refers to these stored paths during the clear. Soft reconfiguration is memory intensive. |
Procedure
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Step 1 |
configure Example:Enters mode. |
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Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
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Step 3 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
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Step 4 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
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Step 5 |
soft-reconfiguration inbound [ always] Example:Configures the software to store updates received from a specified neighbor. Soft reconfiguration inbound causes the software to store the original unmodified route in addition to a route that is modified or filtered. This allows a “soft clear” to be performed after the inbound policy is changed. Soft reconfiguration enables the software to store the incoming updates before apply policy if route refresh is not supported by the peer (otherwise a copy of the update is not stored). The always keyword forces the software to store a copy even when route refresh is supported by the peer. |
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Step 6 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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Log Neighbor Changes
Logging neighbor changes is enabled by default. Use thebgp log neighbor changes disable command to turn off logging. Use the no bgp log neighbor changes disable command to turn logging back on, if it has been disabled.
RP/0/RP0/CPU0:(config)# router bgp 100
RP/0/RP0/CPU0:(config-bgp)# bgp log neighbor changes disable
RP/0/RP0/CPU0:(config)# router bgp 100
RP/0/RP0/CPU0:(config-bgp)# no bgp log neighbor changes disable
BGP Route Reflectors
BGP requires that all iBGP speakers be fully meshed. However, this requirement does not scale well when there are many iBGP speakers. Instead of configuring a confederation, you can reduce the iBGP mesh by using a route reflector configuration. With route reflectors, all iBGP speakers need not be fully meshed because there is a method to pass learned routes to neighbors. In this model, an iBGP peer is configured to be a route reflector responsible for passing iBGP learned routes to a set of iBGP neighbors.
In #concept_3069A67F47124831B884817915F11DF3__ , Router B is configured as a route reflector. When the route reflector receives routes advertised from Router A, it advertises them to Router C, and vice versa. This scheme eliminates the need for the iBGP session between routers A and C.
See BGP Route Reflectors Reference for additional details on route reflectors.
Configure Route Reflector for BGP
Perform this task to configure a route reflector for BGP.
All the neighbors configured with the route-reflector-clientcommand are members of the client group, and the remaining iBGP peers are members of the nonclient group for the local route reflector.
Together, a route reflector and its clients form a cluster. A cluster of clients usually has a single route reflector. In such instances, the cluster is identified by the software as the router ID of the route reflector. To increase redundancy and avoid a single point of failure in the network, a cluster can have more than one route reflector. If it does, all route reflectors in the cluster must be configured with the same 4-byte cluster ID so that a route reflector can recognize updates from route reflectors in the same cluster. The bgp cluster-id command is used to configure the cluster ID when the cluster has more than one route reflector.
Procedure
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Step 1 |
configure Example:Enters mode. |
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Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
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Step 3 |
bgp cluster-id cluster-id Example:Configures the local router as one of the route reflectors serving the cluster. It is configured with a specified cluster ID to identify the cluster. |
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Step 4 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
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Step 5 |
remote-as as-number Example:Creates a neighbor and assigns a remote autonomous system number to it. |
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Step 6 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
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Step 7 |
route-reflector-client Example:Configures the router as a BGP route reflector and configures the neighbor as its client. |
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Step 8 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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BGP Route Reflector: Example
The following example shows how to use an address family to configure internal BGP peer 10.1.1.1 as a route reflector client for unicast prefixes:
router bgp 140
address-family ipv4 unicast
neighbor 10.1.1.1
remote-as 140
address-family ipv4 unicast
route-reflector-client
exit
Configure BGP Route Filtering by Route Policy
Perform this task to configure BGP routing filtering by route policy.
Procedure
| Command or Action | Purpose | |
|---|---|---|
|
Step 1 |
configure Example: |
Enters mode. |
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Step 2 |
route-policy name Example: |
(Optional) Creates a route policy and enters route policy configuration mode, where you can define the route policy. |
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Step 3 |
end-policy Example: |
(Optional) Ends the definition of a route policy and exits route policy configuration mode. |
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Step 4 |
router bgp as-number Example: |
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
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Step 5 |
neighbor ip-address Example: |
Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
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Step 6 |
address-family { ipv4 | ipv6 } unicast Example: |
Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
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Step 7 |
route-policy route-policy-name { in | out } Example: |
Applies the specified policy to inbound routes. |
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Step 8 |
Use the commit or end command. |
commit —Saves the configuration changes and remains within the configuration session.
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Configure BGP Attribute Filtering
The BGP Attribute Filter checks integrity of BGP updates in BGP update messages and optimizes reaction when detecting invalid attributes. BGP Update message contains a list of mandatory and optional attributes. These attributes in the update message include MED, LOCAL_PREF, COMMUNITY, and so on. In some cases, if the attributes are malformed, there is a need to filter these attributes at the receiving end of the router. The BGP Attribute Filter functionality filters the attributes received in the incoming update message. The attribute filter can also be used to filter any attributes that may potentially cause undesirable behavior on the receiving router. Some of the BGP updates are malformed due to wrong formatting of attributes such as the network layer reachability information (NLRI) or other fields in the update message. These malformed updates, when received, causes undesirable behavior on the receiving routers. Such undesirable behavior may be encountered during update message parsing or during re-advertisement of received NLRIs. In such scenarios, its better to filter these corrupted attributes at the receiving end.
The Attribute-filtering is configured by specifying a single or a range of attribute codes and an associated action. When a received Update message contains one or more filtered attributes, the configured action is applied on the message. Optionally, the Update message is also stored to facilitate further debugging and a syslog message is generated on the console. When an attribute matches the filter, further processing of the attribute is stopped and the corresponding action is taken. Perform the following tasks to configure BGP attribute filtering:
Procedure
|
Step 1 |
configure Example:Enters mode. |
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Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
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Step 3 |
attribute-filter group attribute-filter group name Example:Specifies the attribute-filter group name and enters the attribute-filter group configuration mode, allowing you to configure a specific attribute filter group for a BGP neighbor. |
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Step 4 |
attribute attribute code { discard | treat-as-withdraw } Example:
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BGP Next Hop Tracking
BGP receives notifications from the Routing Information Base (RIB) when next-hop information changes (event-driven notifications). BGP obtains next-hop information from the RIB to:
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Determine whether a next hop is reachable.
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Find the fully recursed IGP metric to the next hop (used in the best-path calculation).
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Validate the received next hops.
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Calculate the outgoing next hops.
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Verify the reachability and connectedness of neighbors.
BGP Next Hop Reference provides additional conceptual details on BGP next hop.
Configure BGP Next-Hop Trigger Delay
Perform this task to configure BGP next-hop trigger delay. The Routing Information Base (RIB) classifies the dampening notifications based on the severity of the changes. Event notifications are classified as critical and noncritical. This task allows you to specify the minimum batching interval for the critical and noncritical events.
Procedure
|
Step 1 |
configure Example:Enters mode. |
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Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
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Step 3 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
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Step 4 |
nexthop trigger-delay { critical delay | non-critical delay } Example:Sets the critical next-hop trigger delay. This list provides the default critical and non-critical delay values for the specified address families.
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Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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Disable Next-Hop Processing on BGP Updates
Perform this task to disable next-hop calculation for a neighbor and insert your own address in the next-hop field of BGP updates. Disabling the calculation of the best next hop to use when advertising a route causes all routes to be advertised with the network device as the next hop.
Note |
Next-hop processing can be disabled for address family group, neighbor group, or neighbor address family. |
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
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Step 4 |
remote-as as-number Example:Creates a neighbor and assigns a remote autonomous system number to it. |
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Step 5 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
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Step 6 |
next-hop-self Example:Sets the next-hop attribute for all routes advertised to the specified neighbor to the address of the local router. Disabling the calculation of the best next hop to use when advertising a route causes all routes to be advertised with the local network device as the next hop. |
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Step 7 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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BGP Cost Community
The BGP cost community is a nontransitive extended community attribute that is passed to internal BGP (iBGP) and confederation peers but not to external BGP (eBGP) peers. The cost community feature allows you to customize the local route preference and influence the best-path selection process by assigning cost values to specific routes. The extended community format defines generic points of insertion (POI) that influence the best-path decision at different points in the best-path algorithm.
BGP Cost Community Reference provides additional conceptual details on BGP cost community.
Configure BGP Cost Community
BGP receives multiple paths to the same destination and it uses the best-path algorithm to decide which is the best path to install in RIB. To enable users to determine an exit point after partial comparison, the cost community is defined to tie-break equal paths during the best-path selection process. Perform this task to configure the BGP cost community.
Procedure
|
Step 1 |
configure Example:Enters mode. |
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Step 2 |
route-policy name Example:Enters route policy configuration mode and specifies the name of the route policy to be configured. |
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Step 3 |
set extcommunity cost { cost-extcommunity-set-name | cost-inline-extcommunity-set } [ additive ] Example:Specifies the BGP extended community attribute for cost. |
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Step 4 |
end-policy Example:Ends the definition of a route policy and exits route policy configuration mode. |
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Step 5 |
router bgp as-number Example:Enters BGP configuration mode allowing you to configure the BGP routing process. |
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Step 6 |
Do one of the following:
Applies the cost community to the attach point (route policy). |
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Step 7 |
Do one of the following:
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Step 8 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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Step 9 |
show bgp ip-address Example:Displays the cost community in the following format: Cost: POI : cost-community-ID : cost-number |
Configure BGP Community and Extended-Community Advertisements
Perform this task to specify that community/extended-community attributes should be sent to an eBGP neighbor. These attributes are not sent to an eBGP neighbor by default. By contrast, they are always sent to iBGP neighbors. This section provides examples on how to enable sending community attributes. The send-community-ebgp keyword can be replaced by the send-extended-community-ebgp keyword to enable sending extended-communities.
If the send-community-ebgp command is configured for a neighbor group or address family group, all neighbors using the group inherit the configuration. Configuring the command specifically for a neighbor overrides inherited values.
Note |
BGP community and extended-community filtering cannot be configured for iBGP neighbors. Communities and extended-communities are always sent to iBGP neighbors under VPNv4, MDT, IPv4, and IPv6 address families. |
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
|
Step 4 |
remote-as as-number Example:Creates a neighbor and assigns a remote autonomous system number to it. |
|
Step 5 |
address-family {ipv4 {labeled-unicast | unicast | mdt | | mvpn | rt-filter | tunnel } | ipv6 {labeled-unicast | mvpn | unicast }} Example:Enters neighbor address family configuration mode for the specified address family. Use either ipv4 or ipv6 address family keyword with one of the specified address family sub mode identifiers.
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|
Step 6 |
Use one of these commands:
Example:or Specifies that the router send community attributes or extended community attributes (which are disabled by default for eBGP neighbors) to a specified eBGP neighbor. |
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Step 7 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
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Configuring BGP Large Communities
BGP communities provide a way to group destinations and apply routing decisions such as acceptance, rejection, preference, or redistribution on a group of destinations using community attributes. BGP community attributes are variable length attributes consisting of a set of one or more 4-byte values which are split into two parts of 16 bits. The higher-order 16 bits represents the AS number and the lower order bits represents a locally defined value assigned by the operator of the AS.
Since the adoption of 4-byte ASNs (RFC6793), the BGP communities attribute can no longer accommodate the 4 byte ASNs as you need more than 4 bytes to encode the 4-byte ASN and an AS specific value that you want to tag with the route. Although BGP extended community permits a 4-byte AS to be encoded as the global administrator field, the local administrator field has only 2-byte of available space. So, 6-byte extended community attribute is also unsuitable. To overcome this limitation, you can configure a 12-byte BGP large community which is an optional attribute that provides the most significant 4-byte value to encode autonomous system number as the global administrator and the remaining two 4-byte assigned numbers to encode the local values.
Similar to BGP communities, routers can apply BGP large communities to BGP routes by using route policy languages (RPL) and other routers can then perform actions based on the community that is attached to the route. The policy language provides sets as a container for groups of values for matching purposes.
When large communities are specified in other commands, they are specified as three non negative decimal integers separated by colons. For example, 1:2:3. Each integer is stored in 32 bits. The possible range for each integer is 0 to 4294967295.
In route-policy statements, each integer in the BGP large community can be replaced by any of the following expressions :
-
[x..y] — This expression specifies a range between x and y, inclusive.
-
* —This expression stands for any number.
-
peeras — This expression is replaced by the AS number of the neigbhor from which the community is received or to which the community is sent, as appropriate.
-
not-peeras —This expression matches any number other than the peeras.
-
private-as — This expression specifies any number in the private ASN range: [64512..65534] and [4200000000..4294967294].
These expressions can be also used in policy-match statements.
IOS regular expression (ios-regex) and DFA style regular expression (dfa-regex) can be used in any of the large-community policy match and delete statements. For example, the IOS regular expression ios-regex '^5:.*:7$' is equivalent to the expression 5:*:7.
The send-community-ebgp command is extended to include BGP large communities. This command is required for the BGP speaker to send large communities to ebgp neighbors.
Restrictions and Guidelines
The following restrictions and guidelines apply for BGP large communities:
-
All functionalities of the BGP community attribute is available for the BGP large-community attribute.
-
The send-community-ebgp command is required for the BGP speaker to send large communities to ebgp neighbors.
-
There are no well-known large-communities.
-
The peeras expression cannot be used in a large-community-set.
-
The peeras expression can only be used in large-community match or delete statements that appear in route policies that are applied at the neighbor-in or neighbor-out attach points.
-
The not-peeras expression cannot be used in a large-community-set or in policy set statements.
Configuration Example: Large Community Set
A large-community set defines a set of large communities. Named large-community sets are used in route-policy match and set statements.
This example shows how to create a named large-community set.
RP/0/RP0/CPU0:router(config)# large-community-set catbert
RP/0/RP0/CPU0:router(config-largecomm)# 1: 2: 3,
RP/0/RP0/CPU0:router(config-largecomm)# peeras:2:3
RP/0/RP0/CPU0:router(config-largecomm)# end-set
Configuration Example: Set Large Community
The following example shows how to set the BGP large community attribute in a route, using the set large-community {large-community-set-name | inline-large-community-set | parameter } [additive ] command. You can specify a named large-community-set or an inline set. The additive keyword retains the large communities already present in the route and adds the new set of large communities. However the additive keyword does not result in duplicate entries.
If a particular large community is attached to a route and you specify the same large community again with the additive keyword in the set statement, then the specified large community is not added again. The merging operation removes duplicate entries. This also applies to the peeras keyword.
The peeras expression in the example is replaced by the AS number of the neighbor from which the BGP large community is received or to which the community is sent, as appropriate.
RP/0/RP0/CPU0:router(config)# route-policy mordac
RP/0/RP0/CPU0:router(config-rpl)# set large-community (1:2:3, peeras:2:3)
RP/0/RP0/CPU0:router(config-rpl)# end-set
RP/0/RP0/CPU0:router(config)# large-community-set catbert
RP/0/RP0/CPU0:router(config-largecomm)# 1: 2: 3,
RP/0/RP0/CPU0:router(config-largecomm)# peeras:2:3
RP/0/RP0/CPU0:router(config-largecomm)# end-set
RP/0/RP0/CPU0:router(config)# route-policy wally
RP/0/RP0/CPU0:router(config-rpl)# set large-community catbert additive
RP/0/RP0/CPU0:router(config-rpl)# end-set
In this example, if the route-policy mordac is applied to a neighbor, the ASN of which is 1, then the large community (1:2:3) is set only once.
Note |
You should configure the send-community-ebgp command to send large communities to ebgp neighbors. |
Configuration Example: Large Community Matches-any
The following example shows how to configure a route policy to match any element of a large -community set. This is a boolean condition and returns true if any of the large communities in the route match any of the large communities in the match condition.
RP/0/RP0/CPU0:router(config)# route-policy elbonia
RP/0/RP0/CPU0:router(config-rpl)# if large-community matches-any (1:2:3, 4:5:*) then
RP/0/RP0/CPU0:router(config-rpl)# set local-preference 94
RP/0/RP0/CPU0:router(config-rpl)# endif
RP/0/RP0/CPU0:router(config-rpl)# end-policy
Configuration Example: Large Community Matches-every
The following example shows how to configure a route policy where every match specification in the statement must be matched by at least one large community in the route.
RP/0/RP0/CPU0:router(config)# route-policy bob
RP/0/RP0/CPU0:router(config-rpl)# if large-community matches-every (*:*:3, 4:5:*) then
RP/0/RP0/CPU0:router(config-rpl)# set local-preference 94
RP/0/RP0/CPU0:router(config-rpl)# endif
RP/0/RP0/CPU0:router(config-rpl)# end-policy
In this example, routes with these sets of large communities return TRUE:
-
(1:1:3, 4:5:10)
-
(4:5:3) —This single large community matches both specifications.
-
(1:1:3, 4:5:10, 7:6:5)
Routes with the following set of large communities return FALSE:
(1:1:3, 5:5:10)—The specification (4:5:*) is not matched.
Configuration Example: Large Community Matches-within
The following example shows how to configure a route policy to match within a large community set. This is similar to the large-community matches-any command but every large community in the route must match at least one match specification. Note that if the route has no large communities, then it matches.
RP/0/RP0/CPU0:router(config)# route-policy bob
RP/0/RP0/CPU0:router(config-rpl)# if large-community matches-within (*:*:3, 4:5:*) then
RP/0/RP0/CPU0:router(config-rpl)# set local-preference 103
RP/0/RP0/CPU0:router(config-rpl)# endif
RP/0/RP0/CPU0:router(config-rpl)# end-policy
For example, routes with these sets of large communities return TRUE:
-
(1:1:3, 4:5:10)
-
(4:5:3)
-
(1:2:3, 6:6:3, 9:4:3)
Routes with this set of large communities return FALSE:
(1:1:3, 4:5:10, 7:6:5) —The large community (7:6:5) does not match
Configuration Example: Community Matches-within
The following example shows how to configure a route policy to match within the elements of a community set. This command is similar to the community matches-any command, but every community in the route must match at least one match specification. If the route has no communities, then it matches.
RP/0/RP0/CPU0:router(config)# route-policy bob
RP/0/RP0/CPU0:router(config-rpl)# if community matches-within (*:3, 5:*) then
RP/0/RP0/CPU0:router(config-rpl)# set local-preference 94
RP/0/RP0/CPU0:router(config-rpl)# endif
RP/0/RP0/CPU0:router(config-rpl)# end-policy
For example, routes with these sets of communities return TRUE:
-
(1:3, 5:10)
-
(5:3)
-
(2:3, 6:3, 4:3)
Routes with this set of communities return FALSE:
(1:3, 5:10, 6:5) —The community (6:5) does not match.
Configuration Example: Large Community Is-empty
The following example shows using the large-community is-empty clause to filter routes that do not have the large-community attribute set.
RP/0/RP0/CPU0:router(config)# route-policy lrg_comm_rp4
RP/0/RP0/CPU0:router(config-rpl)# if large-community is-empty then
RP/0/RP0/CPU0:router(config-rpl)# set local-preference 104
RP/0/RP0/CPU0:router(config-rpl)# endif
RP/0/RP0/CPU0:router(config-rpl)# end-policy
Configuration Example: Attribute Filter Group
The following example shows how to configure and apply the attribute-filter group with large-community attributes for a BGP neighbor. The filter specifies the BGP path attributes and an action to take when BGP update message is received. If an update message is received from the BGP neighbor that contains any of the specified attributes, then the specified action is taken. In this example, the attribute filter named dogbert is created and applied to the BGP neighbor 10.0.1.101. It specifies the large community attribute and the action of discard. That means, if the large community BGP path attribute is received in a BGP UPDATE message from the neighbor 10.0.1.101 then the attribute will be discarded before further processing of the message.
RP/0/RP0/CPU0:router(config)# router bgp 100
RP/0/RP0/CPU0:router(config-bgp)# attribute-filter group dogbert
RP/0/RP0/CPU0:router(config-bgp-attrfg)# attribute LARGE-COMMUNITY discard
RP/0/RP0/CPU0:router(config-bgp-attrfg)# neighbor 10.0.1.101
RP/0/RP0/CPU0:router(config-bgp-nbr)# remote-as 6461
RP/0/RP0/CPU0:router(config-bgp-nbr)# update in filtering
RP/0/RP0/CPU0:router(config-nbr-upd-filter)# attribute-filter group dogbert
Configuration Example: Deleting Large Community
The following example shows how to delete specified BGP large-communities from a route policy using the delete large-community command.
RP/0/RP0/CPU0:router(config)# route-policy lrg_comm_rp2
RP/0/RP0/CPU0:router(config-rpl)# delete large-community in (ios-regex '^100000:’)
RP/0/RP0/CPU0:router(config-rpl)# delete large-community all
RP/0/RP0/CPU0:router(config-rpl)# delete large-community not in (peeras:*:*, 41289:*:*)
Verification
This example displays the routes with large-communities given in the show bgp large-community list-of-large-communities [exact-match ] command. If the optional keyword exact-match is used, then the listed routes will contain only the specified large communities. Otherwise, the displayed routes may contain additional large communities.
RP/0/0/CPU0:R1# show bgp large-community 1:2:3 5:6:7
Thu Mar 23 14:40:33.597 PDT
BGP router identifier 4.4.4.4, local AS number 3
BGP generic scan interval 60 secs
Non-stop routing is enabled
BGP table state: Active
Table ID: 0xe0000000 RD version: 66
BGP main routing table version 66
BGP NSR Initial initsync version 3 (Reached)
BGP NSR/ISSU Sync-Group versions 66/0
BGP scan interval 60 secs
Status codes: s suppressed, d damped, h history, * valid, > best
i - internal, r RIB-failure, S stale, N Nexthop-discard
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
* 10.0.0.3/32 10.10.10.3 0 94 0 ?
* 10.0.0.5/32 10.11.11.5 0 0 5 ?
This example displays the large community attached to a network using the show bgp ip-address/ prefix-length command.
RP/0/0/CPU0:R4# show bgp 10.3.3.3/32
Thu Mar 23 14:36:15.301 PDT
BGP routing table entry for 10.3.3.3/32
Versions:
Process bRIB/RIB SendTblVer
Speaker 42 42
Last Modified: Mar 22 20:04:46.000 for 18:31:30
Paths: (1 available, best #1)
Advertised to peers (in unique update groups):
10.11.11.5
Path #1: Received by speaker 0
Advertised to peers (in unique update groups):
10.11.11.5
Local
10.10.10.3 from 10.10.10.3 (10.3.3.3)
Origin incomplete, metric 0, localpref 94, valid, internal, best, group-best
Received Path ID 0, Local Path ID 0, version 42
Community: 258:259 260:261 262:263 264:265
Large Community: 1:2:3 5:6:7 4123456789:4123456780:4123456788Redistribute iBGP Routes into IGP
Perform this task to redistribute iBGP routes into an Interior Gateway Protocol (IGP), such as Intermediate System-to-Intermediate System (IS-IS) or Open Shortest Path First (OSPF).
Note |
Use of the bgp redistribute-internal command requires the clear route * command to be issued to reinstall all BGP routes into the IP routing table. |
 Caution |
Redistributing iBGP routes into IGPs may cause routing loops to form within an autonomous system. Use this command with caution. |
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
bgp redistribute-internal Example:Allows the redistribution of iBGP routes into an IGP, such as IS-IS or OSPF. |
|
Step 4 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Redistribute IGPs to BGP
Perform this task to configure redistribution of a protocol into the VRF address family.
Even if Interior Gateway Protocols (IGPs) are used as the PE-CE protocol, the import logic happens through BGP. Therefore, all IGP routes have to be imported into the BGP VRF table.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
vrf vrf-name Example:Enables BGP routing for a particular VRF on the PE router. |
|
Step 4 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
|
Step 5 |
Do one of the following:
Example:Configures redistribution of a protocol into the VRF address family context. The redistribute command is used if BGP is not used between the PE-CE routers. If BGP is used between PE-CE routers, the IGP that is used has to be redistributed into BGP to establish VPN connectivity with other PE sites. Redistribution is also required for inter-table import and export. |
|
Step 6 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Update Groups
The BGP Update Groups feature contains an algorithm that dynamically calculates and optimizes update groups of neighbors that share outbound policies and can share the update messages. The BGP Update Groups feature separates update group replication from peer group configuration, improving convergence time and flexibility of neighbor configuration.
Monitor BGP Update Groups
This task displays information related to the processing of BGP update groups.
Procedure
|
show bgp [ ipv4 { unicast | multicast | all | tunnel } | ipv6 { unicast | all } | all { unicast | multicast | all labeled-unicast | tunnel } | vpnv4 unicast | vrf { vrf-name | all } [ ipv4 unicast ipv6 unicast ] | vpvn6 unicast ] update-group [ neighbor ip-address | process-id.index [ summary | performance-statistics ]] Example:Displays information about BGP update groups.
|
Displaying BGP Update Groups: Example
The following is sample output from the show bgp update-group command run in EXEC configurationXR EXEC mode:
show bgp update-group
Update group for IPv4 Unicast, index 0.1:
Attributes:
Outbound Route map:rm
Minimum advertisement interval:30
Messages formatted:2, replicated:2
Neighbors in this update group:
10.0.101.92
Update group for IPv4 Unicast, index 0.2:
Attributes:
Minimum advertisement interval:30
Messages formatted:2, replicated:2
Neighbors in this update group:
10.0.101.91
L3VPN iBGP PE-CE
The L3VPN iBGP PE-CE feature helps establish an iBGP (internal Border Gateway Protocol) session between the provider edge (PE) and customer edge (CE) devices to exchange BGP routing information. A BGP session between two BGP peers is said to be an iBGP session if the BGP peers are in the same autonomous systems.
Restrictions for L3VPN iBGP PE-CE
-
When the iBGP PE CE feature is toggled and the neighbor no longer supports route-refresh or soft-reconfiguration inbound, a manual session flap must be done to see the change. When this occurs, the following message is displayed: RP/0/RP0/CPU0: %ROUTING-BGP-5-CFG_CHG_RESET: Internal VPN client configuration change on neighbor 10.10.10.1 requires HARD reset (clear bgp 10.10.10.1) to take effect.
-
iBGP PE CE CLI configuration is not available for peers under default-VRF, except for neighbor/session-group.
-
This feature does not work on regular VPN clients (eBGP VPN clients).
-
Attributes packed inside the ATTR_SET reflects changes made by the inbound route-policy on the iBGP CE and does not reflect the changes made by the export route-policy for the specified VRF.
-
Different VRFs of the same VPN (that is, in different PE routers) that are configured with iBGP PE-CE peering sessions must use different Route Distinguisher (RD) values under respective VRFs. The iBGP PE CE feature does ot work if the RD values are the same for the ingress and egress VRF.
Configuring L3VPN iBGP PE-CE
L3VPN iBGP PE-CE can be enabled on the neighbor, neighbor-group, or session-group. To configure L3VPN iBGP PE-CE, follow these steps:
Before you begin
The CE must be an internal BGP peer.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
vrf vrf-name Example:Configures a VRF instance. |
|
Step 4 |
neighbor ip-address internal-vpn-client Example:Configures a CE neighboring device with which to exchange routing information. The neighbor internal-vpn-client command stacks the iBGP-CE neighbor path in the VPN attribute set. |
|
Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
|
Step 6 |
show bgp vrf vrf-name neighbors ip-address Displays whether the iBGP PE-CE feature is enabled for the VRF CE peer, or not. |
|
Step 7 |
show bgp { vpnv4| vpnv6 } unicast rd Displays the ATTR_SET attributes in the command output when the L3VPN iBGP PE-CE is enabled on a CE. |
Example
R1(config-bgp-vrf-nbr)#neighbor 10.10.10.1 ?
. . .
internal-vpn-client Preserve iBGP CE neighbor path in ATTR_SET across VPN core
. . .
R1(config-bgp-vrf-nbr)#neighbor 10.10.10.1 internal-vpn-client
router bgp 65001
bgp router-id 100.100.100.2
address-family ipv4 unicast
address-family vpnv4 unicast
!
vrf ce-ibgp
rd 65001:100
address-family ipv4 unicast
!
neighbor 10.10.10.1
remote-as 65001
internal-vpn-client
The following is an example of the output of the show bgp vrf vrf-name neighbors ip-address command when the L3VPN iBGP PE-CE is enabled on a CE peer:
R1#show bgp vrf ce-ibgp neighbors 10.10.10.1
BGP neighbor is 10.10.10.1, vrf ce-ibgp
Remote AS 65001, local AS 65001, internal link
Remote router ID 100.100.100.1
BGP state = Established, up for 00:00:19
. . .
Multi-protocol capability received
Neighbor capabilities:
Route refresh: advertised (old + new) and received (old + new)
4-byte AS: advertised and received
Address family IPv4 Unicast: advertised and received
CE attributes will be preserved across the core
Received 2 messages, 0 notifications, 0 in queue
Sent 2 messages, 0 notifications, 0 in queue
. . .
The following is an example of the output of the show bgp vpn4/vpn6 unicast rd command when the L3VPN iBGP PE-CE is enabled on a CE peer:
BGP routing table entry for 1.1.1.0/24, Route Distinguisher: 200:300
Versions:
Process bRIB/RIB SendTblVer
Speaker 10 10
Last Modified: Aug 28 13:11:17.000 for 00:01:00
Paths: (1 available, best #1)
Advertised to update-groups (with more than one peer):
0.2
Path #1: Received by speaker 0
Advertised to update-groups (with more than one peer):
0.2
Local, (Received from a RR-client)
20.20.20.2 from 20.20.20.2 (100.100.100.2)
Received Label 24000
Origin IGP, localpref 100, valid, internal, best, group-best, import-candidate,
not-in-vrf Received Path ID 0, Local Path ID 1, version 10
Extended community: RT:228:237
ATTR-SET [
Origin-AS: 200
AS-Path: 51320 52325 59744 12947 21969 50346 18204 36304 41213 23906 33646
Origin: incomplete
Metric: 204
Local-Pref: 234
Aggregator: 304 34.3.3.3
Atomic Aggregator
Community: 1:60042 2:41661 3:47008 4:9280 5:39778 6:1069 7:15918 8:8994 9:52701
10:10268 11:26276 12:8506 13:7131 14:65464 15:14304 16:33615 17:54991 18:40149 19:19401
Extended community: RT:100:1 RT:1.1.1.1:1]
Flow-tag propagation
The flow-tag propagation feature enables you to establish a co-relation between route-policies and user-policies. Flow-tag propagation using BGP allows user-side traffic-steering based on routing attributes such as, AS number, prefix lists, community strings and extended communities. Flow-tag is a logical numeric identifier that is distributed through RIB as one of the routing attribute of FIB entry in the FIB lookup table. A flow-tag is instantiated using the 'set' operation from RPL and is referenced in the C3PL PBR policy, where it is associated with actions (policy-rules) against the flow-tag value.
You can use flow-tag propagation to:
-
Classify traffic based on destination IP addresses (using the Community number) or based on prefixes (using Community number or AS number).
-
Select a TE-group that matches the cost of the path to reach a service-edge based on customer site service level agreements (SLA).
-
Apply traffic policy (TE-group selection) for specific customers based on SLA with its clients.
-
Divert traffic to application or cache server.
Restrictions for Flow-Tag Propagation
Some restrictions are placed with regard to using Quality-of-service Policy Propagation Using Border Gateway Protocol (QPPB) and flow-tag feature together. These include:
- A route-policy can have either 'set qos-group' or 'set flow-tag,' but not both for a prefix-set.
- Route policy for qos-group and route policy flow-tag cannot have overlapping routes. The QPPB and flow tag features can coexist (on same as well as on different interfaces) as long as the route policy used by them do not have any overlapping route.
- Mixing usage of qos-group and flow-tag in route-policy and policy-map is not recommended.
Source and destination-based flow tag
The source-based flow tag feature allows you to match packets based on the flow-tag assigned to the source address of the incoming packets. Once matched, you can then apply any supported PBR action on this policy.
Configure Source and Destination-based Flow Tag
This task applies flow-tag to a specified interface. The packets are matched based on the flow-tag assigned to the source address of the incoming packets.
Note |
You will not be able to enable both QPPB and flow tag feature simultaneously on an interface. |
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
interface type interface-path-id Example:Enters interface configuration mode and associates one or more interfaces to the VRF. |
|
Step 3 |
ipv4 | ipv6 bgp policy propagation input flow-tag{destination | source} Example:Enables flow-tag policy propagation on source or destination IP address on an interface. |
|
Step 4 |
Use the commit or end command. commit —Saves the configuration changes, and remains within the configuration session.
|
Example
show running-config interface gigabitEthernet 0/0/0/12
Thu Feb 12 01:51:37.820 UTC
interface GigabitEthernet0/0/0/12
service-policy type pbr input flowMatchPolicy
ipv4 bgp policy propagation input flow-tag source
ipv4 address 192.5.1.2 255.255.255.0
!
RP/0/RSP0/CPU0:ASR9K-0#show running-config policy-map type pbr flowMatchPolicy
Thu Feb 12 01:51:45.776 UTC
policy-map type pbr flowMatchPolicy
class type traffic flowMatch36
transmit
!
class type traffic flowMatch38
transmit
!
class type traffic class-default
!
end-policy-map
!
RP/0/RSP0/CPU0:ASR9K-0#show running-config class-map type traffic flowMatch36
Thu Feb 12 01:52:04.838 UTC
class-map type traffic match-any flowMatch36
match flow-tag 36
end-class-map
!
BGP Keychains
BGP keychains enable keychain authentication between two BGP peers. The BGP endpoints must both comply with draft-bonica-tcp-auth-05.txt and a keychain on one endpoint and a password on the other endpoint does not work.
BGP is able to use the keychain to implement hitless key rollover for authentication. Key rollover specification is time based, and in the event of clock skew between the peers, the rollover process is impacted. The configurable tolerance specification allows for the accept window to be extended (before and after) by that margin. This accept window facilitates a hitless key rollover for applications (for example, routing and management protocols).
The key rollover does not impact the BGP session, unless there is a keychain configuration mismatch at the endpoints resulting in no common keys for the session traffic (send or accept).
Configure Keychains for BGP
Keychains provide secure authentication by supporting different MAC authentication algorithms and provide graceful key rollover. Perform this task to configure keychains for BGP. This task is optional.
Note |
If a keychain is configured for a neighbor group or a session group, a neighbor using the group inherits the keychain. Values of commands configured specifically for a neighbor override inherited values. |
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
|
Step 4 |
remote-as as-number Example:Creates a neighbor and assigns a remote autonomous system number to it. |
|
Step 5 |
keychain name Example:Configures keychain-based authentication. |
|
Step 6 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Master Key Tuple Configuration
This feature specifies TCP Authentication Option (TCP-AO), which replaces the TCP MD5 option. TCP-AO uses the Message Authentication Codes (MACs), which provides the following:
-
Protection against replays for long-lived TCP connections
-
More details on the security association with TCP connections than TCP MD5
-
A larger set of MACs with minimal other system and operational changes
Note |
TCPAO and TCP MD5 are never permitted to be used simultaneously. TCP-AO supports IPv6, and is fully compatible with the proposed requirements for the replacement of TCP MD5. |
-
keychain configuration
-
tcp ao keychain configuration
The system translates each key, such "key_id" that is under a keychain, as MKT. The keychain configuration owns part of the configuration like secret, lifetimes, and algorithms. While the "tcp ao keychain" mode owns the TCP AO-specific configuration for an MKT (send_id and receive_id).
Keychain Configurations
Configuration Guidelines
In order to run a successful configuration, ensure that you follow the configuration guidelines:
-
An allowed value range for both Send_ID and Receive_ID is 0 to 255.
-
You can link only one keychain to an application neighbor.
-
Under the same keychain, if you configure the same send_id key again under the keys that have an overlapping lifetime, then the old key becomes unusable until you correct the configuration.
-
The system sends a warning message in the following scenarios: - If there is a change in Send_ID or Receive_ID.
-
If the corresponding key is currently active, and is in use by some connection.
-
BGP neighbor can ONLY use one of the authentication options: -
MD5
-
EA
-
AO
Note
If you configure one of these options, the system rejects the other authentication options during the configuration time.
-
Configuration Guidelines for TCP AO BGP Neighbor
The configuration guidelines are:
-
Configure all the necessary configurations (key_string, MAC_algorithm, send_lifetime, accept_lifetime, send_id, receive_id) under key_id with the desired lifetime it wants to use the key_id for.
-
Configure a matching MKT in the peer side with exactly same lifetime.
-
Once a keychain-key is linked to tcp-ao, do not change the components of the key. If you want TCP to consider another key for use, you can configure that dynamically. Based on the ‘start-time’of send lifetime, TCP AO uses the key.
-
Send_ID and Receive_ID under a key_id (under a keychain) must have the same lifetime range. For example, send-lifetime==accept-lifetime.
TCP considers only expiry of send-lifetime to transition to next active key and it does not consider accept-lifetime at all.
-
Do not configure a key with send-lifetime that is covered by another key’s send-lifetime.
For example, if there is a key that is already configured with send-lifetime of “04:00:00 November 01, 2017 07:00:00 November 01, 2017” and the user now configures another key with send-lifetime of “05:00:00 November 01, 2017 06:00:00 November 01, 2017”, this might result into connection flap.
TCP AO tries to transition back to the old key once the new key is expired. However, if the new key has already expired, TCP AO can’t use it, which might result in segment loss and hence connection flap.
-
Configure minimum of 15 minutes of overlapping time between the two overlapping keys. When a key expires, TCP does not use it and hence out-of-order segments with that key are dropped.
-
We recommend configuring send_id and receive_id to be same for a key_id for simplicity.
-
TCP does not have any restriction on the number of keychains and keys under a keychain. The system does not support more than 4000 keychains, any number higher than 4000 might result in unexpected behaviors.
Keychain Configuration
key chain <keychain_name>
key <key_id>
accept-lifetime <start-time> <end-time>
key-string <master-key>
send-lifetime <start-time> <end-time>
cryptographic-algorithm <algorithm>
!
!
TCP Configuration
tcp ao
keychain <keychain_name1>
key-id <key_id> send_id <0-255> receive_id <0-255>
!
tcp ao
keychain bgp_ao
key 0 SendID 0 ReceiveID 0
key 1 SendID 1 ReceiveID 1
key 2 SendID 3 ReceiveID 4
!
keychain ldp_ao
key 1 SendID 100 ReceiveID 200
key 120 SendID 1 ReceiveID 1
!
BGP Configurations
Applications like BGP provide the tcp-ao keychain and related information that it uses per neighbor. Following are the optional configurations per tcp-ao keychain:
-
include-tcp-options
-
accept-non-ao-connections
router bgp <AS-number>
neighbor <neighbor-ip>
remote-as <remote-as-number>
ao <keychain-name> include-tcp-options enable/disable <accept-ao-mismatch-connections>
!
XML Configurations
BGP XML
TCP-AO XML
<?xml version="1.0" encoding="UTF-8"?>
<Request>
<Set>
<Configuration>
<IP_TCP>
<AO>
<Enable>
true
</Enable>
<KeychainTable>
<Keychain>
<Naming>
<Name> bgp_ao_xml </Name>
</Naming>
<Enable>
true
</Enable>
<KeyTable>
<Key>
<Naming>
<KeyID> 0 </KeyID>
</Naming>
<SendID> 0 </SendID>
<ReceiveID> 0 </ReceiveID>
</Key>
</KeyTable>
</Keychain>
</KeychainTable>
</AO>
</IP_TCP>
</Configuration>
</Set>
<Commit/>
</Request>BGP Nonstop Routing
The Border Gateway Protocol (BGP) Nonstop Routing (NSR) with Stateful Switchover (SSO) feature enables all bgp peerings to maintain the BGP state and ensure continuous packet forwarding during events that could interrupt service. Under NSR, events that might potentially interrupt service are not visible to peer routers. Protocol sessions are not interrupted and routing states are maintained across process restarts and switchovers.
BGP Nonstop Routing Reference for additional details.
Configure BGP Nonstop Routing
Note |
In some scenarios, it is possible that some or all bgp sessions are not NSR-READY. The |
Disable BGP Nonstop Routing
Perform this task to disable BGP Nonstop Routing (NSR):
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the BGP AS number, and enters the BGP configuration mode, for configuring BGP routing processes. |
|
Step 3 |
nsr disable Example:Disables BGP Nonstop routing. |
|
Step 4 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Disable BGP Nonstop Routing: Example
The following example shows how to disable BGP NSR:
configure
router bgp 120
no nsr
end
Re-enable BGP Nonstop Routing
If BGP Nonstop Routing (NSR) is disabled, use the following steps to turn BGP NSR back on using the following steps:
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the BGP AS number, and enters the BGP configuration mode, for configuring BGP routing processes. |
|
Step 3 |
no nsr disable Example:Enables BGP Nonstop routing. |
|
Step 4 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Re-enable BGP Nonstop Routing: Example
The following example shows how to enable BGP NSR:
configure
router bgp 120
nsr
end
Accumulated Interior Gateway Protocol Attribute
The Accumulated Interior Gateway Protocol (AiGP)Attribute is an optional non-transitive BGP Path Attribute. The attribute type code for the AiGP Attribute is to be assigned by IANA. The value field of the AiGP Attribute is defined as a set of Type/Length/Value elements (TLVs). The AiGP TLV contains the Accumulated IGP Metric.
The AiGP feature is required in the 3107 network to simulate the current OSPF behavior of computing the distance associated with a path. OSPF/LDP carries the prefix/label information only in the local area. Then, BGP carries the prefix/lable to all the remote areas by redistributing the routes into BGP at area boundaries. The routes/labels are then advertised using LSPs. The next hop for the route is changed at each ABR to local router which removes the need to leak OSPF routes across area boundaries. The bandwidth available on each of the core links is mapped to OSPF cost, hence it is imperative that BGP carries this cost correctly between each of the PEs. This functionality is achieved by using the AiGP.
Originate Prefixes with AiGP
Perform this task to configure origination of routes with the AiGP metric:
Before you begin
Origination of routes with the accumulated interior gateway protocol (AiGP) metric is controlled by configuration. AiGP attributes are attached to redistributed routes that satisfy following conditions:
-
The protocol redistributing the route is enabled for AiGP.
-
The route is an interior gateway protocol (iGP) route redistributed into border gateway protocol (BGP). The value assigned to the AiGP attribute is the value of iGP next hop to the route or as set by a route-policy.
-
The route is a static route redistributed into BGP. The value assigned is the value of next hop to the route or as set by a route-policy.
-
The route is imported into BGP through network statement. The value assigned is the value of next hop to the route or as set by a route-policy.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
route-policy aigp_policy Example:Enters route-policy configuration mode and sets the route-policy |
|
Step 3 |
set aigp-metric igp-cost Example:Sets the internal routing protocol cost as the aigp metric. |
|
Step 4 |
exit Example:Exits route-policy configuration mode. |
|
Step 5 |
router bgp as-number Example:Specifies the BGP AS number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 6 |
address-family {ipv4 | ipv6 } unicast Example:Specifies either the IPv4 or IPv6 address family and enters address family configuration submode. |
|
Step 7 |
redistribute ospf osp route-policy plcy_name metric value Example:Allows the redistribution of AiBGP metric into OSPF. |
|
Step 8 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Originating Prefixes With AiGP: Example
The following is a sample configuration for originating prefixes with the AiGP metric attribute:
route-policy aigp-policy
set aigp-metric 4
set aigp-metric igp-cost
end-policy
!
router bgp 100
address-family ipv4 unicast
network 10.2.3.4/24 route-policy aigp-policy
redistribute ospf osp1 metric 4 route-policy aigp-policy
!
!
end
Configure BGP Accept Own
The BGP Accept Own feature allows you to handle self-originated VPN routes, which a BGP speaker receives from a route-reflector (RR). A 'self-originated' route is one which was originally advertized by the speaker itself. As per BGP protocol [RFC4271], a BGP speaker rejects advertisements that were originated by the speaker itself. However, the BGP Accept Own mechanism enables a router to accept the prefixes it has advertised, when reflected from a route-reflector that modifies certain attributes of the prefix. A special community called ACCEPT-OWN is attached to the prefix by the route-reflector, which is a signal to the receiving router to bypass the ORIGINATOR_ID and NEXTHOP/MP_REACH_NLRI check. Generally, the BGP speaker detects prefixes that are self-originated through the self-origination check (ORIGINATOR_ID, NEXTHOP/MP_REACH_NLRI) and drops the received updates. However, with the Accept Own community present in the update, the BGP speaker handles the route.
One of the applications of BGP Accept Own is auto-configuration of extranets within MPLS VPN networks. In an extranet configuration, routes present in one VRF is imported into another VRF on the same PE. Normally, the extranet mechanism requires that either the import-rt or the import policy of the extranet VRFs be modified to control import of the prefixes from another VRF. However, with Accept Own feature, the route-reflector can assert that control without the need for any configuration change on the PE. This way, the Accept Own feature provides a centralized mechanism for administering control of route imports between different VRFs.
Note |
BGP Accept Own is supported only for VPNv4 and VPNv6 address families in neighbor configuration mode. |
Perform this task to configure BGP Accept Own:
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
|
Step 4 |
remote-as as-number Example:Assigns a remote autonomous system number to the neighbor. |
|
Step 5 |
update-source type interface-path-id Example:Allows sessions to use the primary IP address from a specific interface as the local address when forming a session with a neighbor. |
|
Step 6 |
address-family {vpnv4 unicast | vpnv6 unicast } Example:Specifies the address family as VPNv4 or VPNv6 and enters neighbor address family configuration mode. |
|
Step 7 |
accept-own [inheritance-disable ] Example:Enables handling of self-originated VPN routes containing Accept_Own community. Use the inheritance-disable keyword to disable the "accept own" configuration and to prevent inheritance of "acceptown" from a parent configuration. |
BGP Accept Own Configuration: Example
-
PE11 is configured with Customer VRF and Service VRF.
-
OSPF is used as the IGP.
-
VPNv4 unicast and VPNv6 unicast address families are enabled between the PE and RR neighbors and IPv4 and IPv6 are enabled between PE and CE neighbors.
-
CE1 originates prefix X.
-
Prefix X is installed in customer VRF as (RD1:X).
-
Prefix X is advertised to IntraAS-RR11 as (RD1:X, RT1).
-
IntraAS-RR11 advertises X to InterAS-RR1 as (RD1:X, RT1).
-
InterAS-RR1 attaches RT2 to prefix X on the inbound and ACCEPT_OWN community on the outbound and advertises prefix X to IntraAS-RR31.
-
IntraAS-RR31 advertises X to PE11.
-
PE11 installs X in Service VRF as (RD2:X,RT1, RT2, ACCEPT_OWN).
This example shows how to configure BGP Accept Own on a PE router.
router bgp 100
neighbor 45.1.1.1
remote-as 100
update-source Loopback0
address-family vpnv4 unicast
route-policy pass-all in
accept-own
route-policy drop_111.x.x.x out
!
address-family vpnv6 unicast
route-policy pass-all in
accept-own
route-policy drop_111.x.x.x out
!
!router bgp 100
neighbor 45.1.1.1
remote-as 100
update-source Loopback0
address-family vpnv4 unicast
route-policy rt_stitch1 in
route-reflector-client
route-policy add_bgp_ao out
!
address-family vpnv6 unicast
route-policy rt_stitch1 in
route-reflector-client
route-policy add_bgp_ao out
!
!
extcommunity-set rt cs_100:1
100:1
end-set
!
extcommunity-set rt cs_1001:1
1001:1
end-set
!
route-policy rt_stitch1
if extcommunity rt matches-any cs_100:1 then
set extcommunity rt cs_1000:1 additive
endif
end-policy
!
route-policy add_bgp_ao
set community (accept-own) additive
end-policy
!
BGP Link-State
BGP Link-State (LS) is an Address Family Identifier (AFI) and Sub-address Family Identifier (SAFI) originally defined to carry interior gateway protocol (IGP) link-state information through BGP. The BGP Network Layer Reachability Information (NLRI) encoding format for BGP-LS and a new BGP Path Attribute called the BGP-LS attribute are defined in RFC7752. The identifying key of each Link-State object, namely a node, link, or prefix, is encoded in the NLRI and the properties of the object are encoded in the BGP-LS attribute.
Note |
IGPs do not use BGP LS data from remote peers. BGP does not download the received BGP LS data to any other component on the router. |
An example of a BGP-LS application is the Segment Routing Path Computation Element (SR-PCE). The SR-PCE can learn the SR capabilities of the nodes in the topology and the mapping of SR segments to those nodes. This can enable the SR-PCE to perform path computations based on SR-TE and to steer traffic on paths different from the underlying IGP-based distributed best-path computation.
The following figure shows a typical deployment scenario. In each IGP area, one or more nodes (BGP speakers) are configured with BGP-LS. These BGP speakers form an iBGP mesh by connecting to one or more route-reflectors. This way, all BGP speakers (specifically the route-reflectors) obtain Link-State information from all IGP areas (and from other ASes from eBGP peers).
Exchange Link State Information with BGP Neighbor
The following example shows how to exchange link-state information with a BGP neighbor:
Router# configure
Router(config)# router bgp 1
Router(config-bgp)# neighbor 10.0.0.2
Router(config-bgp-nbr)# remote-as 1
Router(config-bgp-nbr)# address-family link-state link-state
Router(config-bgp-nbr-af)# exit
IGP Link-State Database Distribution
A given BGP node may have connections to multiple, independent routing domains. IGP link-state database distribution into BGP-LS is supported for both OSPF and IS-IS protocols in order to distribute this information on to controllers or applications that desire to build paths spanning or including these multiple domains.
To distribute OSPFv2 link-state data using BGP-LS, use the distribute link-state command in router configuration mode.
Router# configure
Router(config)# router ospf 100
Router(config-ospf)# distribute link-state instance-id 32
Usage Guidelines and Limitations
-
BGP-LS supports IS-IS and OSPFv2.
-
The identifier field of BGP-LS (referred to as the Instance-ID) identifies the IGP routing domain where the NLRI belongs. The NLRIs representing link-state objects (nodes, links, or prefixes) from the same IGP routing instance must use the same Instance-ID value.
-
When there is only a single protocol instance in the network where BGP-LS is operational, we recommend configuring the Instance-ID value to 0.
-
Assign consistent BGP-LS Instance-ID values on all BGP-LS Producers within a given IGP domain.
-
NLRIs with different Instance-ID values are considered to be from different IGP routing instances.
-
Unique Instance-ID values must be assigned to routing protocol instances operating in different IGP domains. This allows the BGP-LS Consumer (for example, SR-PCE) to build an accurate segregated multi-domain topology based on the Instance-ID values, even when the topology is advertised via BGP-LS by multiple BGP-LS Producers in the network.
-
If the BGP-LS Instance-ID configuration guidelines are not followed, a BGP-LS Consumer may see duplicate link-state objects for the same node, link, or prefix when there are multiple BGP-LS Producers deployed. This may also result in the BGP-LS Consumers getting an inaccurate network-wide topology.
-
The following table defines the supported extensions to the BGP-LS address family for carrying IGP topology information (including SR information) via BGP. For more information on the BGP-LS TLVs, refer to Border Gateway Protocol - Link State (BGP-LS) Parameters.
| TLV Code Point | Description | Produced by IS-IS | Produced by OSPFv2 | Produced by BGP |
|---|---|---|---|---|
| 256 | Local Node Descriptors | X | X | — |
| 257 | Remote Node Descriptors | X | X | — |
| 258 | Link Local/Remote Identifiers | X | X | — |
| 259 | IPv4 interface address | X | X | — |
| 260 | IPv4 neighbor address | X | ||
| 261 | IPv6 interface address | X | — | — |
| 262 | IPv6 neighbor address | X | — | — |
| 263 | Multi-Topology ID | X | — | — |
| 264 | OSPF Route Type | — | X | — |
| 265 | IP Reachability Information | X | X | — |
| 266 | Node MSD TLV | X | X | — |
| 267 | Link MSD TLV | X | X | — |
| 512 | Autonomous System | — | — | X |
| 513 | BGP-LS Identifier | — | — | X |
| 514 | OSPF Area-ID | — | X | — |
| 515 | IGP Router-ID | X | X | — |
| 516 | BGP Router-ID TLV | — | — | X |
| 517 | BGP Confederation Member TLV | — | — | X |
| 1024 | Node Flag Bits | X | X | — |
| 1026 | Node Name | X | X | — |
| 1027 | IS-IS Area Identifier | X | — | — |
| 1028 | IPv4 Router-ID of Local Node | X | X | — |
| 1029 | IPv6 Router-ID of Local Node | X | — | — |
| 1030 | IPv4 Router-ID of Remote Node | X | X | — |
| 1031 | IPv6 Router-ID of Remote Node | X | — | — |
| 1034 | SR Capabilities TLV | X | X | — |
| 1035 | SR Algorithm TLV | X | X | — |
| 1036 | SR Local Block TLV | X | X | — |
| 1039 | Flex Algo Definition (FAD) TLV | X | X | — |
| 1044 | Flex Algorithm Prefix Metric (FAPM) TLV | X | X | — |
| 1088 | Administrative group (color) | X | X | — |
| 1089 | Maximum link bandwidth | X | X | — |
| 1090 | Max. reservable link bandwidth | X | X | — |
| 1091 | Unreserved bandwidth | X | X | — |
| 1092 | TE Default Metric | X | X | — |
| 1093 | Link Protection Type | X | X | — |
| 1094 | MPLS Protocol Mask | X | X | — |
| 1095 | IGP Metric | X | X | — |
| 1096 | Shared Risk Link Group | X | X | — |
| 1099 | Adjacency SID TLV | X | X | — |
| 1100 | LAN Adjacency SID TLV | X | X | — |
| 1101 | PeerNode SID TLV | — | — | X |
| 1102 | PeerAdj SID TLV | — | — | X |
| 1103 | PeerSet SID TLV | — | — | X |
| 1114 | Unidirectional Link Delay TLV | X | X | — |
| 1115 | Min/Max Unidirectional Link Delay TLV | X | X | — |
| 1116 | Unidirectional Delay Variation TLV | X | X | — |
| 1117 | Unidirectional Link Loss | X | X | — |
| 1118 | Unidirectional Residual Bandwidth | X | X | — |
| 1119 | Unidirectional Available Bandwidth | X | X | — |
| 1120 | Unidirectional Utilized Bandwidth | X | X | — |
| 1122 | Application-Specific Link Attribute TLV | X | X | — |
| 1152 | IGP Flags | X | X | — |
| 1153 | IGP Route Tag | X | X | — |
| 1154 | IGP Extended Route Tag | X | — | — |
| 1155 | Prefix Metric | X | X | — |
| 1156 | OSPF Forwarding Address | — | X | — |
| 1158 | Prefix-SID | X | X | — |
| 1159 | Range | X | X | — |
| 1161 | SID/Label TLV | X | X | — |
| 1170 | Prefix Attribute Flags | X | X | — |
| 1171 | Source Router Identifier | X | — | — |
| 1172 | L2 Bundle Member Attributes TLV | X | — | — |
| 1173 | Extended Administrative Group | X | X | — |
Configure BGP Link-state
To exchange BGP link-state (LS) information with a BGP neighbor, perform these steps:
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the BGP AS number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
address-family link-state link-state Example:Distributes BGP link-state information to the specified neighbor. |
|
Step 4 |
neighbor ip-address Example:Configures a CE neighbor. The ip-address argument must be a private address. |
|
Step 5 |
remote-as as-number Example:Configures the remote AS for the CE neighbor. |
|
Step 6 |
address-family link-state link-state Example:Distributes BGP link-state information to the specified neighbor. |
|
Step 7 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Example
router bgp 100
address-family link-state link-state
!
neighbor 10.0.0.2
remote-as 1
address-family link-state link-state
Configure Domain Distinguisher
To configure unique identifier four-octet ASN, perform these steps:
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the BGP AS number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
address-family link-state link-state Example:Enters address-family link-state configuration mode. |
|
Step 4 |
domain-distinguisher unique-id Example:Configures unique identifier four-octet ASN. Range is from 1 to 4294967295. |
|
Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
BGP Permanent Network
The permanent-network command in global address family configuration mode uses a route-policy to identify the set of prefixes (networks) for which permanent paths is to be configured. The advertise permanent-network command in neighbor address-family configuration mode is used to identify the peers to whom the permanent paths must be advertised. The permanent paths is always advertised to peers having the advertise permanent-network configuration, even if a different best-path is available. The permanent path is not advertised to peers that are not configured to receive permanent path.
The permanent network feature supports only prefixes in IPv4 unicast and IPv6 unicast address-families under the default Virtual Routing and Forwarding (VRF).
Restrictions
These restrictions apply while configuring the permanent network:
-
Permanent network prefixes must be specified by the route-policy on the global address family.
-
You must configure the permanent network with route-policy in global address family configuration mode and then configure it on the neighbor address family configuration mode.
-
When removing the permanent network configuration, remove the configuration in the neighbor address family configuration mode and then remove it from the global address family configuration mode.
Configure BGP Permanent Network
Perform this task to configure BGP permanent network. You must configure at least one route-policy to identify the set of prefixes (networks) for which the permanent network (path) is to be configured.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
prefix-set prefix-set-name Example:Enters prefix set configuration mode and defines a prefix set for contiguous and non-contiguous set of bits. |
|
Step 3 |
exit Example:Exits prefix set configuration mode and enters global configuration mode. |
|
Step 4 |
route-policy route-policy-name Example:Creates a route policy and enters route policy configuration mode, where you can define the route policy. |
|
Step 5 |
end-policy Example:Ends the definition of a route policy and exits route policy configuration mode. |
|
Step 6 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode. |
|
Step 7 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. |
|
Step 8 |
permanent-network route-policy route-policy-name Example:Configures the permanent network (path) for the set of prefixes as defined in the route-policy. |
|
Step 9 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
|
Step 10 |
show bgp {ipv4 | ipv6} unicast prefix-set Example:(Optional) Displays whether the prefix-set is a permanent network in BGP. |
Advertise Permanent Network
Perform this task to identify the peers to whom the permanent paths must be advertised.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode. |
|
Step 3 |
neighbor ip-address Example:Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer. |
|
Step 4 |
remote-as as-number Example:Assigns the neighbor a remote autonomous system number. |
|
Step 5 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. |
|
Step 6 |
advertise permanent-network Example:Specifies the peers to whom the permanent network (path) is advertised. |
|
Step 7 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
|
Step 8 |
show bgp {ipv4 | ipv6} unicast neighbor ip-address Example:(Optional) Displays whether the neighbor is capable of receiving BGP permanent networks. |
Enable BGP Unequal Cost Recursive Load Balancing
Procedure
| Command or Action | Purpose | |
|---|---|---|
|
Step 1 |
configure Example: |
Enters mode. |
|
Step 2 |
router bgp as-number Example: |
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
address-family { ipv4 | ipv6 } unicast Example: |
Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
|
Step 4 |
maximum-paths { ebgp | ibgp | eibgp } maximum [ unequal-cost ] Example: |
Configures the maximum number of parallel routes that BGP installs in the routing table.
When eiBGP is applied, eBGP or iBGP load balancing cannot be configured; however, eBGP and iBGP load balancing can coexist. |
|
Step 5 |
exit Example: |
Exits the current configuration mode. |
|
Step 6 |
neighbor ip-address Example: |
Configures a CE neighbor. The ip-address argument must be a private address. |
|
Step 7 |
dmz-link-bandwidth Example: |
Originates a demilitarized-zone (DMZ) link-bandwidth extended community for the link to an eBGP/iBGP neighbor. |
|
Step 8 |
Use the commit or end command. |
commit —Saves the configuration changes and remains within the configuration session.
|
DMZ Link Bandwidth for Unequal Cost Recursive Load Balancing
The demilitarized zone (DMZ) link bandwidth for unequal cost recursive load balancing feature provides support for unequal cost load balancing for recursive prefixes on local node using DMZ link bandwidth. Use the dmz-link-bandwidth command in BGP neighbor configuration mode and the bandwidth command in interface configuration mode to The unequal load balance is achieved.
When the PE router includes the link bandwidth extended community in its updates to the remote PE through the Multiprotocol Interior BGP (MP-iBGP) session (either IPv4 or VPNv4), the remote PE automatically does load balancing if the maximum-paths command is enabled.
Note |
Unequal cost recursive load balancing happens across maximum eight paths only. |
Enable BGP Unequal Cost Recursive Load Balancing
Procedure
| Command or Action | Purpose | |
|---|---|---|
|
Step 1 |
configure Example: |
Enters mode. |
|
Step 2 |
router bgp as-number Example: |
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
address-family { ipv4 | ipv6 } unicast Example: |
Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
|
Step 4 |
maximum-paths { ebgp | ibgp | eibgp } maximum [ unequal-cost ] Example: |
Configures the maximum number of parallel routes that BGP installs in the routing table.
When eiBGP is applied, eBGP or iBGP load balancing cannot be configured; however, eBGP and iBGP load balancing can coexist. |
|
Step 5 |
exit Example: |
Exits the current configuration mode. |
|
Step 6 |
neighbor ip-address Example: |
Configures a CE neighbor. The ip-address argument must be a private address. |
|
Step 7 |
dmz-link-bandwidth Example: |
Originates a demilitarized-zone (DMZ) link-bandwidth extended community for the link to an eBGP/iBGP neighbor. |
|
Step 8 |
Use the commit or end command. |
commit —Saves the configuration changes and remains within the configuration session.
|
DMZ Link Bandwidth Over EBGP Peer
The demilitarized zone (DMZ) link bandwidth extended community is an optional non-transitive attribute; therefore, it is not advertised to eBGP peers by default but it is advertised only to iBGP peers. This extended community is meant for load balancing over multi-paths. However, Cisco IOS-XR enables advertising of the DMZ link bandwidth to an eBGP peer, or receiving the DMZ link bandwidth by an eBGP peer. This feature also gives the user the option to send the bandwidth unchanged, or take the accumulated bandwidth over all the egress links and advertise that to the upstream eBGP peer.
Use the ebgp-send-community-dmz command to send the community to eBGP peers. By default, the link bandwidth extended-community attribute associated with the best path is sent.
When the cumulative keyword is used, the value of the link bandwidth extended community is set to the sum of link bandwidth values of all the egress-multipaths. If the DMZ link bandwidth value of the multipaths is unknown, for instance, some paths do not have that attribute, then unequal cost load-balancing is not done at that node. However, the sum of the known DMZ link bandwidth values is calculated and sent to the eBGP peer.
Use the ebgp-recv-community-dmz command to receive the community from eBGP peers.
Note |
The ebgp-send-community-dmz and ebgp-recv-community-dmz commands can be configured in the neigbor, neighbour-group, and session-group configuration mode. |
Use the bgp bestpath as-path multipath-relax and bgp bestpath as-path ignore commands to handle multipath across different autonomous systems.
Sending and Receiving DMZ Link Bandwidth Extended Community over eBGP Peer
Procedure
|
Step 1 |
configure Example:Enters mode. |
||
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
||
|
Step 3 |
neighbor ip-address Example:Enters the neighbor configuration mode for configuring BGP routing sessions. |
||
|
Step 4 |
ebgp-send-extcommunity-dmz ip-address Example:Sends the DMZ link bandwidth extended community to the eBGP neighbor.
|
||
|
Step 5 |
exit Example:Exits the neighbor configuration mode and enters into BGP configuration mode. |
||
|
Step 6 |
neighbor ip-address Example:Enters the neighbor configuration mode for configuring BGP routing sessions. |
||
|
Step 7 |
ebgp-recv-extcommunity-dmz Example:Receives the DMZ link bandwidth extended community to the eBGP neighbor. |
||
|
Step 8 |
exit Example:Exits the neighbor configuration mode and enters into BGP configuration mode. |
DMZ Link Bandwidth: Example
The following examples shows how Router R1 sends DMZ link bandwidth extended communities to Router R2 over eBGP peer connection:
R1: sending router
------------------
neighbour 10.3.3.3
remote-as 2
ebgp-send-extcommunity-dmz
address-family ipv4 unicast
route-policy pass in
route-policy pass out
!
R2: Receiving router
--------------------
neighbor 192.0.2.1
remote-as 3
ebgp-recv-extcommunity-dmz
address-family ipv4 unicast
route-policy pass in
!
route-policy pass out
!
The following is a sample configuration that displays the DMZ link bandwidth configuration in the sending (R1) router:
RP/0/RP0/CPU0:router)# show bgp ipv4 unicast 10.1.1.1/32 detail
Path #1: Received by speaker 0
Flags: 0x4000000001040003, import: 0x20
Advertised to update-groups (with more than one peer):
0.4
Advertised to peers (in unique update groups):
20.0.0.1
3
11.1.0.2 from 11.1.0.2 (11.1.0.2)
Origin incomplete, metric 20, localpref 100, valid, external, best, group-best
Received Path ID 0, Local Path ID 0, version 21
Extended community: LB:3:192
Origin-AS validity: not-found
The following is a sample configuration that displays DMZ link bandwidth configuration in the receiving (R2) router:
RP/0/RP0/CPU0:router)# show bgp ipv4 unicast 10.1.1.1/32 detail
Paths: (1 available, best #1)
Not advertised to any peer
Path #1: Received by speaker 0
Not advertised to any peer
1 3
20.0.0.2 from 20.0.0.2 (10.0.0.81)
Origin incomplete, localpref 100, valid, external, best, group-best
Received Path ID 0, Local Path ID 0, version 17
Extended community: LB:1:192
Origin-AS validity: not-found
BGP Prefix Origin Validation using RPKI
A BGP route associates an address prefix with a set of autonomous systems (AS) that identify the interdomain path the prefix has traversed in the form of BGP announcements. This set is represented as the AS_PATH attribute in BGP and starts with the AS that originated the prefix.
To help reduce well-known threats against BGP including prefix mis-announcing and monkey-in-the-middle attacks, one of the security requirements is the ability to validate the origination AS of BGP routes. The AS number claiming to originate an address prefix (as derived from the AS_PATH attribute of the BGP route) needs to be verified and authorized by the prefix holder.
The Resource Public Key Infrastructure (RPKI) is an approach to build a formally verifiable database of IP addresses and AS numbers as resources. The RPKI is a globally distributed database containing, among other things, information mapping BGP (internet) prefixes to their authorized origin-AS numbers. Routers running BGP can connect to the RPKI to validate the origin-AS of BGP paths.
Configure RPKI Cache-server
Perform this task to configure Resource Public Key Infrastructure (RPKI) cache-server parameters.
Configure the RPKI cache-server parameters in rpki-server configuration mode. Use the rpki server command in router BGP configuration mode to enter into the rpki-server configuration mode
Procedure
|
Step 1 |
configure Example:Enters mode. |
||
|
Step 2 |
router bgp as-number Example:Specifies the BGP AS number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
||
|
Step 3 |
rpki cache {host-name | ip-address } Example:Enters rpki-server configuration mode and enables configuration of RPKI cache parameters. |
||
|
Step 4 |
Use one of these commands:
Example:Or Specifies a transport method for the RPKI cache.
|
||
|
Step 5 |
(Optional) username user_name Example:Specifies a (SSH) username for the RPKI cache-server. |
||
|
Step 6 |
(Optional) password Example:Specifies a (SSH) password for the RPKI cache-server.
|
||
|
Step 7 |
preference preference_value Example:Specifies a preference value for the RPKI cache. Range for the preference value is 1 to 10. Setting a lower preference value is better. |
||
|
Step 8 |
purge-time time Example:Configures the time BGP waits to keep routes from a cache after the cache session drops. Set purge time in seconds. Range for the purge time is 30 to 360 seconds. |
||
|
Step 9 |
Use one of these commands.
Example:Or Configures the time BGP waits in between sending periodic serial queries to the cache. Set refresh-time in seconds. Range for the refresh time is 15 to 3600 seconds. Configure the off option to specify not to send serial-queries periodically. |
||
|
Step 10 |
Use one these commands.
Example:Or Configures the time BGP waits for a response after sending a serial or reset query. Set response-time in seconds. Range for the response time is 15 to 3600 seconds. Configure the off option to wait indefinitely for a response. |
||
|
Step 11 |
shutdown Example:Configures shut down of the RPKI cache. |
||
|
Step 12 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Configure BGP Prefix Validation
Starting from Release 6.5.1, RPKI is disabled by default. From Release 6.5.1, use the following task to configure RPKI Prefix Validation.
Router(config)# router bgp 100
/* The bgp origin-as validation time and bgp origin-as validity signal ibgp commands are optional. */.
Router(config-bgp)# bgp origin-as validation time 50
Router(config-bgp)# bgp origin-as validation time off
Router(config-bgp)# bgp origin-as validation signal ibgp
Router(config-bgp)# address-family ipv4 unicast
Router(config-bgp-af)# bgp origin-as validation enable
Use the following commands to verify the origin-as validation configuration:
Router# show bgp origin-as validity
Thu Mar 14 04:18:09.656 PDT
BGP router identifier 10.1.1.1, local AS number 1
BGP generic scan interval 60 secs
Non-stop routing is enabled
BGP table state: Active
Table ID: 0xe0000000 RD version: 514
BGP main routing table version 514
BGP NSR Initial initsync version 2 (Reached)
BGP NSR/ISSU Sync-Group versions 0/0
BGP scan interval 60 secs
Status codes: s suppressed, d damped, h history, * valid, > best
i - internal, r RIB-failure, S stale, N Nexthop-discard
Origin codes: i - IGP, e - EGP, ? - incomplete
Origin-AS validation codes: V valid, I invalid, N not-found, D disabled
Network Next Hop Metric LocPrf Weight Path
*> 209.165.200.223/27 0.0.0.0 0 32768 ?
*> 209.165.200.225/27 0.0.0.0 0 32768 ?
*> 19.1.2.0/24 0.0.0.0 0 32768 ?
*> 19.1.3.0/24 0.0.0.0 0 32768 ?
*> 10.1.2.0/24 0.0.0.0 0 32768 ?
*> 10.1.3.0/24 0.0.0.0 0 32768 ?
*> 10.1.4.0/24 0.0.0.0 0 32768 ?
*> 198.51.100.1/24 0.0.0.0 0 32768 ?
*> 203.0.113.235/24 0.0.0.0 0 32768 ?
V*> 209.165.201.0/27 10.1.2.1 0 4002 i
N*> 198.51.100.2/24 10.1.2.1 0 4002 i
I*> 198.51.100.1/24 10.1.2.1 0 4002 i
*> 192.0.2.1.0/24 0.0.0.0 0 32768 ?
Router# show bgp process
Mon Jul 9 16:47:39.428 PDT
BGP Process Information:
...
Use origin-AS validity in bestpath decisions
Allow (origin-AS) INVALID paths
Signal origin-AS validity state to neighbors
Address family: IPv4 Unicast
...
Origin-AS validation is enabled for this address-family
Use origin-AS validity in bestpath decisions for this address-family
Allow (origin-AS) INVALID paths for this address-family
Signal origin-AS validity state to neighbors with this address-family
Configure RPKI Bestpath Computation
Perform this task to configure RPKI bestpath computation options.
Procedure
|
Step 1 |
configure Example:Enters mode. |
||
|
Step 2 |
router bgp as-number Example:Specifies the BGP AS number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
||
|
Step 3 |
rpki bestpath use origin-as validity Example:Enables the validity states of BGP paths to affect the path's preference in the BGP bestpath process. This configuration can also be done in router BGP address family submode. |
||
|
Step 4 |
rpki bestpath origin-as allow invalid Example:Allows all "invalid" paths to be considered for BGP bestpath computation.
This configuration takes effect only when the rpki bestpath use origin-as validity configuration is enabled. |
||
|
Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Resilient Per-CE Label Allocation Mode
|
Feature Name |
Release Information |
Feature Description |
|
Per-Prefix Label Allocation Support on BVI |
Release 7.3.1 |
You can configure connected routes and static routes in per-prefix mode on the BVI. However, dynamic protocols such as BGP in per-prefix mode on the BVI is not supported. |
-
No support for load balancing across CEs
-
Temporary forwarding loop during local traffic diversion to support PIC
-
No support for EIBGP multipath load balancing
-
Forwarding performance impact
In the Resilient Per-CE label allocation scheme, BGP installs a unique rewrite label in LSD for every unique set of CE paths or next hops. There may be one or more prefixes in BGP table that points to this label. BGP also installs the CE paths (primary) and optionally a backup PE path into RIB. FIB learns about the label rewrite information from LSD and the IP paths from RIB. In steady state, labeled traffic destined to the resilient per-CE label is load balanced across all the CE next hops. When all the CE paths fail, any traffic destined to that label will result in an IP lookup and will be forwarded towards the backup PE path, if available. This action is performed on the label independently of the number of prefixes that may point to the label, resulting in the PIC behavior during primary paths failure.
Configure Resilient Per-CE Label Allocation Mode Under VRF Address Family
Perform this task to configure resilient per-ce label allocation mode under VRF address family.
Procedure
|
Step 1 |
configure Example:Enters global configuration mode. |
|
Step 2 |
router bgpas-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
vrfvrf-instance Example:Configures a VRF instance. |
|
Step 4 |
address-family {ipv4 | ipv6} unicast Example:Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. |
|
Step 5 |
label-mode per-ce Example:Configures resilient per-ce label allocation mode. |
|
Step 6 |
Do one of the following:
Example:or Saves configuration changes.
|
This example shows how to configure resilient per-ce label allocation mode under VRF address family:
RP/0/RP0/CPU0:router# configure
RP/0/RP0/CPU0:router(config)# router bgp 666
RP/0/RP0/CPU0:router(config-bgp)# vrf vrf-pe
RP/0/RP0/CPU0:router(config-bgp-vrf)# address-family ipv4 unicast
RP/0/RP0/CPU0:router(config-bgp-vrf-af)# label mode per-ce
RP/0/RP0/CPU0:router(config-bgp-vrf-af)# endConfigure Resilient Per-CE Label Allocation Mode Using Route-Policy
Perform this task to configure resilient per-ce label allocation mode using a route-policy.
Procedure
|
Step 1 |
configure Example:Enters global configuration mode. |
|
Step 2 |
route-policypolicy-name Example:Creates a route policy and enters route policy configuration mode. |
|
Step 3 |
set label-mode per-ce Example:Configures resilient per-ce label allocation mode. |
|
Step 4 |
Do one of the following:
Example:or Saves configuration changes.
|
This example shows how to configure resilient per-ce label allocation mode using a route-policy:
RP/0/RP0/CPU0:router# configure
RP/0/RP0/CPU0:router(config)# route-policy route1
RP/0/RP0/CPU0:router(config-rpl)# set label-mode per-ce
RP/0/RP0/CPU0:router(config-rpl)# end
This example shows how to configure route-policy to vrf :
RP/0/RP0/CPU0:(config)# router bgp 666
RP/0/RP0/CPU0:(config-bgp)# vrf vrf-pe
RP/0/RP0/CPU0:(config-bgp-vrf)# address-family ipv4 unicast
RP/0/RP0/CPU0:(config-bgp-vrf-af)# label mode route-policy route1
BGP VRF Dynamic Route Leaking
The Border Gateway Protocol (BGP) dynamic route leaking feature provides the ability to import routes between the default-vrf (Global VRF) and any other non-default VRF, to provide connectivity between a global and a VPN host. The import process installs the Internet route in a VRF table or a VRF route in the Internet table, providing connectivity.
The dynamic route leaking is enabled by:
-
Importing from default-VRF to non-default-VRF, using the import from default-vrf route-policy route-policy-name [ advertise-as-vpn] command in VRF address-family configuration mode.
If the advertise-as-vpn option is configured, the paths imported from the default-VRF to the non-default-VRF are advertised to the PEs as well as to the CEs. If the advertise-as-vpn option is not configured, the paths imported from the default-VRF to the non-default-VRF are not advertised to the PE. However, the paths are still advertised to the CEs.
-
Importing from non-default-VRF to default VRF, using the export to default-vrf route-policy route-policy-name command in VRF address-family configuration mode.
A route-policy is mandatory to filter the imported routes. This reduces the risk of unintended import of routes between the Internet table and the VRF tables and the corresponding security issues. There is no hard limit on the number of prefixes that can be imported. The import creates a new prefix in the destination VRF, which increases the total number of prefixes and paths. However, each VRF importing global routes adds workload equivalent to a neighbor receiving the global table. This is true even if the user filters out all but a few prefixes. Hence, importing five to ten VRFs is ideal.
Configure VRF Dynamic Route Leaking
Before you begin
A route-policy is mandatory for configuring dynamic route leaking. Use the route-policy route-policy-name command in global configuration mode to configure a route-policy.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
vrf vrf_name Example:Enters VRF configuration mode. |
|
Step 3 |
address-family {ipv4 | ipv6} unicast Example:Enters VRF address-family configuration mode. |
|
Step 4 |
Use one of these options:
Example:or
|
|
Step 5 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
VRF Dynamic Route Leaking Configuration: Example
vrf vrf_1
address-family ipv6 unicast
import from default-vrf route-policy rpl_dynamic_route_import
!
end
vrf vrf_1
address-family ipv6 unicast
export to default-vrf route-policy rpl_dynamic_route_export
!
end
What to do next
These show bgp command output displays information from the dynamic route leaking configuration:
-
Use the show bgp prefix command to display the source-RD and the source-VRF for imported paths, including the cases when IPv4 or IPv6 unicast prefixes have imported paths.
-
Use the show bgp imported-routes command to display IPv4 unicast and IPv6 unicast address-families under the default-VRF.
Configuring a VPN Routing and Forwarding Instance in BGP
Layer 3 (virtual private network) VPN can be configured only if there is an available Layer 3 VPN license for the line card slot on which the feature is being configured. If advanced IP license is enabled, 4096 Layer 3 VPN routing and forwarding instances (VRFs) can be configured on an interface. If the infrastructure VRF license is enabled, eight Layer 3 VRFs can be configured on the line card.
RP/0/RP0/CPU0:router#LC/0/0/CPU0:Dec 15 17:57:53.653 : rsi_agent[247]:
%LICENSE-ASR9K_LICENSE-2-INFRA_VRF_NEEDED : 5 VRF(s) are configured without license A9K-iVRF-LIC in violation of the Software Right To Use Agreement.
This feature may be disabled by the system without the appropriate license.
Contact Cisco to purchase the license immediately to avoid potential service interruption.Note |
An AIP license is not required for configuring L2VPN services. |
The following tasks are used to configure a VPN routing and forwarding (VRF) instance in BGP:
Define Virtual Routing and Forwarding Tables in Provider Edge Routers
Perform this task to define the VPN routing and forwarding (VRF) tables in the provider edge (PE) routers.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
vrf vrf-name Example:Configures a VRF instance. |
|
Step 3 |
address-family { ipv4 | ipv6 } unicast Example:Specifies either the IPv4 or IPv6 address family and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
|
Step 4 |
maximum prefix maximum [ threshold ] Example:Configures a limit to the number of prefixes allowed in a VRF table. A maximum number of routes is applicable to dynamic routing protocols as well as static or connected routes. You can specify a threshold percentage of the prefix limit using the mid-threshold argument. |
|
Step 5 |
import route-policy policy-name Example:(Optional) Provides finer control over what gets imported into a VRF. This import filter discards prefixes that do not match the specified policy-name argument. |
|
Step 6 |
import route-target [ as-number : nn | ip-address : nn ] Example:Specifies a list of route target (RT) extended communities. Only prefixes that are associated with the specified import route target extended communities are imported into the VRF. |
|
Step 7 |
export route-policy policy-name Example:(Optional) Provides finer control over what gets exported into a VRF. This export filter discards prefixes that do not match the specified policy-name argument. |
|
Step 8 |
export route-target [ as-number : nn | ip-address : nn ] Example:Specifies a list of route target extended communities. Export route target communities are associated with prefixes when they are advertised to remote PEs. The remote PEs import them into VRFs which have import RTs that match these exported route target communities. |
|
Step 9 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Configure Route Distinguisher
The route distinguisher (RD) makes prefixes unique across multiple VPN routing and forwarding (VRF) instances.
In the L3VPN multipath same route distinguisher (RD)environment, the determination of whether to install a prefix in RIB or not is based on the prefix's bestpath. In a rare misconfiguration situation, where the best pah is not a valid path to be installed in RIB, BGP drops the prefix and does not consider the other paths. The behavior is different for different RD setup, where the non-best multipath will be installed if the best multipath is invalid to be installed in RIB.
Perform this task to configure the RD.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Enters BGP configuration mode allowing you to configure the BGP routing process. |
|
Step 3 |
bgp router-id ip-address Example:Configures a fixed router ID for the BGP-speaking router. |
|
Step 4 |
vrf vrf-name Example:Configures a VRF instance. |
|
Step 5 |
rd { as-number : nn | ip-address : nn | auto } Example:Configures the route distinguisher. Use the auto keyword if you want the router to automatically assign a unique RD to the VRF. Automatic assignment of RDs is possible only if a router ID is configured using the bgp router-id command in router configuration mode. This allows you to configure a globally unique router ID that can be used for automatic RD generation. The router ID for the VRF does not need to be globally unique, and using the VRF router ID would be incorrect for automatic RD generation. Having a single router ID also helps in checkpointing RD information for BGP graceful restart, because it is expected to be stable across reboots. |
|
Step 6 |
Do one of the following:
Example:or Saves configuration changes.
|
Configure PE-PE or PE-RR Interior BGP Sessions
To enable BGP to carry VPN reachability information between provider edge (PE) routers you must configure the PE-PE interior BGP (iBGP) sessions. A PE uses VPN information carried from the remote PE router to determine VPN connectivity and the label value to be used so the remote (egress) router can demultiplex the packet to the correct VPN during packet forwarding.
The PE-PE, PE-route reflector (RR) iBGP sessions are defined to all PE and RR routers that participate in the VPNs configured in the PE router.
Perform this task to configure PE-PE iBGP sessions and to configure global VPN options on a PE.
Procedure
|
Step 1 |
configure Example:Enters mode. |
|
Step 2 |
router bgp as-number Example:Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
|
Step 3 |
address-family vpnv4 unicast Example:Enters VPN address family configuration mode. |
|
Step 4 |
exit Example:Exits the current configuration mode. |
|
Step 5 |
neighbor ip-address Example:Configures a PE iBGP neighbor. |
|
Step 6 |
remote-as as-number Example:Assigns the neighbor a remote autonomous system number. |
|
Step 7 |
description text Example:(Optional) Provides a description of the neighbor. The description is used to save comments and does not affect software function. |
|
Step 8 |
password { clear | encrypted } password Example:Enables Message Digest 5 (MD5) authentication on the TCP connection between the two BGP neighbors. |
|
Step 9 |
shutdown Example:Terminates any active sessions for the specified neighbor and removes all associated routing information. |
|
Step 10 |
timers keepalive hold-time Example:Set the timers for the BGP neighbor. |
|
Step 11 |
update-source type interface-id Example:Allows iBGP sessions to use the primary IP address from a specific interface as the local address when forming an iBGP session with a neighbor. |
|
Step 12 |
address-family vpnv4 unicast Example:Enters VPN neighbor address family configuration mode. |
|
Step 13 |
route-policy route-policy-name in Example:Specifies a routing policy for an inbound route. The policy can be used to filter routes or modify route attributes. |
|
Step 14 |
route-policy route-policy-name out Example:Specifies a routing policy for an outbound route. The policy can be used to filter routes or modify route attributes. |
|
Step 15 |
Use the commit or end command. commit —Saves the configuration changes and remains within the configuration session.
|
Configure BGP as PE-CE Protocol
Perform this task to configure BGP on the PE and establish PE-CE communication using BGP.
Procedure
| Command or Action | Purpose | |||
|---|---|---|---|---|
|
Step 1 |
configure Example: |
Enters mode. |
||
|
Step 2 |
router bgp as-number Example: |
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process. |
||
|
Step 3 |
vrf vrf-name Example: |
Enables BGP routing for a particular VRF on the PE router. |
||
|
Step 4 |
bgp router-id ip-address Example: |
Configures a fixed router ID for a BGP-speaking router. |
||
|
Step 5 |
label-allocation-mode per-ce Example: |
|
||
|
Step 6 |
address-family { ipv4 | ipv6 } unicast Example: |
Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
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|
Step 7 |
network { ip-address / prefix-length | ip-address mask } Example: |
Originates a network prefix in the address family table in the VRF context. |
||
|
Step 8 |
aggregate-address address / mask-length Example: |
Configures aggregation in the VRF address family context to summarize routing information to reduce the state maintained in the core. This summarization introduces some inefficiency in the PE edge, because an additional lookup is required to determine the ultimate next hop for a packet. When configured, a summary prefix is advertised instead of a set of component prefixes, which are more specifics of the aggregate. The PE advertises only one label for the aggregate. Because component prefixes could have different next hops to CEs, an additional lookup has to be performed during data forwarding. |
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|
Step 9 |
exit Example: |
Exits the current configuration mode. |
||
|
Step 10 |
neighbor ip-address Example: |
Configures a CE neighbor. The ip-address argument must be a private address. |
||
|
Step 11 |
remote-as as-number Example: |
Configures the remote AS for the CE neighbor. |
||
|
Step 12 |
password { clear | encrypted } password Example: |
Enable Message Digest 5 (MD5) authentication on a TCP connection between two BGP neighbors. |
||
|
Step 13 |
ebgp-multihop [ ttl-value ] Example: |
Configures the CE neighbor to accept and attempt BGP connections to external peers residing on networks that are not directly connected. |
||
|
Step 14 |
Do one of the following:
Example: |
Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode. To see a list of all the possible keywords and arguments for this command, use the CLI help (?). |
||
|
Step 15 |
site-of-origin [ as-number : nn | ip-address : nn ] Example: |
Configures the site-of-origin (SoO) extended community. Routes that are learned from this CE neighbor are tagged with the SoO extended community before being advertised to the rest of the PEs. SoO is frequently used to detect loops when as-override is configured on the PE router. If the prefix is looped back to the same site, the PE detects this and does not send the update to the CE. |
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|
Step 16 |
as-override Example: |
Configures AS override on the PE router. This causes the PE router to replace the CE’s ASN with its own (PE) ASN.
|
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|
Step 17 |
allowas-in [ as-occurrence-number ] Example: |
Allows an AS path with the PE autonomous system number (ASN) a specified number of times. Hub and spoke VPN networks need the looping back of routing information to the HUB PE through the HUB CE. When this happens, due to the presence of the PE ASN, the looped-back information is dropped by the HUB PE. To avoid this, use the allowas-in command to allow prefixes even if they have the PEs ASN up to the specified number of times. |
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|
Step 18 |
route-policy route-policy-name in Example: |
Specifies a routing policy for an inbound route. The policy can be used to filter routes or modify route attributes. |
||
|
Step 19 |
route-policy route-policy-name out Example: |
Specifies a routing policy for an outbound route. The policy can be used to filter routes or modify route attributes. |
||
|
Step 20 |
Use the commit or end command. |
commit —Saves the configuration changes and remains within the configuration session.
|
Resetting an eBGP Session Immediately Upon Link Failure
By default, if a link goes down, all BGP sessions of any directly adjacent external peers are immediately reset. Use the bgp fast-external-fallover disable command to disable automatic resetting. Turn the automatic reset back on using the no bgp fast-external-fallover disable command.
RP/0/RP0/CPU0:router#configure
RP/0/RP0/CPU0:router(config)#lpts pifib hardware police location 0/2/CPU0
RP/0/RP0/CPU0:router(config-pifib-policer-per-node)#flow bgp configured rate 4000
RP/0/RP0/CPU0:router(config-pifib-policer-per-node)#flow bgp known rate 4000
RP/0/RP0/CPU0:router(config-pifib-policer-per-node)#flow bgp default rate 4000
RP/0/RP0/CPU0:router(config-pifib-policer-per-node)#commitInformation about Implementing BGP
To implement BGP, you need to understand the following concepts:
BGP Router Identifier
For BGP sessions between neighbors to be established, BGP must be assigned a router ID. The router ID is sent to BGP peers in the OPEN message when a BGP session is established.
BGP attempts to obtain a router ID in the following ways (in order of preference):
-
By means of the address configured using the bgp router-id command in router configuration mode.
-
By using the highest IPv4 address on a loopback interface in the system if the router is booted with saved loopback address configuration.
-
By using the primary IPv4 address of the first loopback address that gets configured if there are not any in the saved configuration.
If none of these methods for obtaining a router ID succeeds, BGP does not have a router ID and cannot establish any peering sessions with BGP neighbors. In such an instance, an error message is entered in the system log, and the show bgp summary command displays a router ID of 0.0.0.0. After BGP has obtained a router ID, it continues to use it even if a better router ID becomes available. This usage avoids unnecessary flapping for all BGP sessions. However, if the router ID currently in use becomes invalid (because the interface goes down or its configuration is changed), BGP selects a new router ID (using the rules described) and all established peering sessions are reset.
Note |
We strongly recommend that the bgp router-id command is configured to prevent unnecessary changes to the router ID (and consequent flapping of BGP sessions). |
BGP Attributes and Operators
This table summarizes the BGP attributes and operators per attach points.
|
Attach Point |
Attribute |
Match |
Set |
|---|---|---|---|
|
aggregation |
as-path |
in is-local length neighbor-is originates-from passes-through unique-length |
— |
|
as-path-length |
is, ge, le, eq |
— | |
|
as-path-unique-length |
is, ge, le, eq |
— | |
|
community |
is-empty matches-any matches-every |
set set additive delete in delete not in delete all |
|
|
destination |
in |
— | |
|
extcommunity cost |
— |
set set additive |
|
|
local-preference |
is, ge, le, eq |
set |
|
|
med |
is, eg, ge, le |
setset +set - |
|
|
next-hop |
in |
set |
|
|
origin |
is |
set |
|
|
source |
in |
— | |
|
suppress-route |
— |
suppress-route |
|
|
weight |
— |
set |
|
|
allocate-label |
as-path |
in is-local length neighbor-is originates-from passes-through unique-length |
— |
|
as-path-length |
is, ge, le, eq |
— | |
|
as-path-unique-length |
is, ge, le, eq |
— | |
|
community |
is-empty matches-any matches-every |
— | |
|
destination |
in |
— | |
|
label |
— |
set |
|
|
local-preference |
is, ge, le, eq |
— | |
|
med |
is, eg, ge, le |
— | |
|
next-hop |
in |
— | |
|
origin |
is |
— | |
|
source |
in |
— | |
|
clear-policy |
as-path |
in is-local length neighbor-is originates-from passes-through unique-length |
— |
|
as-path-length |
is, ge, le, eq |
— | |
|
as-path-unique-length |
is, ge, le, eq |
— | |
|
dampening |
as-path |
in is-local length neighbor-is originates-from passes-through unique-length |
— |
|
as-path-length |
is, ge, le, eq |
— | |
|
as-path-unique-length |
is, ge, le, eq |
— | |
|
community |
is-empty matches-any matches-every |
— | |
|
dampening |
—/ |
set dampening |
|
|
destination |
in |
— | |
|
local-preference |
is, ge, le, eq |
— | |
|
med |
is, eg, ge, le |
— | |
|
next-hop |
in |
— | |
|
origin |
is |
— | |
|
source |
in |
— | |
|
debug |
destination |
in |
— |
|
default originate |
med |
— |
set set + set - |
|
rib-has-route |
in |
— | |
|
neighbor-in |
as-path |
in is-local length NA neighbor-is originates-from passes-through unique-length |
prepend prepend most-recent remove as-path private-as replace |
|
as-path-length |
is, ge, le, eq |
— | |
|
as-path-unique-length |
is, ge, le, eq |
— | |
|
communitycommunity with ‘peeras’ |
is-empty matches-any matches-every |
set set additive delete-in delete-not-in delete-all |
|
|
destination |
in |
— | |
|
extcommunity cost |
— |
set set additive |
|
|
extcommunity rt |
is-empty matches-any matches-every matches-within |
set additive delete-in delete-not-in delete-all |
|
|
extcommunity soo |
is-empty matches-any matches-every matches-within |
— | |
|
local-preference |
is, ge, le, eq |
set |
|
|
med |
is, eg, ge, le |
set set + set - |
|
|
next-hop |
in |
set set peer address |
|
|
origin |
is |
set |
|
|
route-aggregated |
route-aggregated |
NA |
|
|
source |
in |
— | |
|
weight |
— |
set |
|
|
neighbor-out |
as-path |
in is-local length — neighbor-is originates-from passes-through unique-length |
prepend prepend most-recent remove as-path private-as replace |
|
as-path-length |
is, ge, le, eq |
— | |
|
as-path-unique-length |
is, ge, le, eq |
— | |
|
communitycommunity with ‘peeras’ |
is-empty matches-any matches-every |
set set additive delete-in delete-not-in delete-all |
|
|
destination |
in |
— | |
|
extcommunity cost |
— |
set set additive |
|
|
extcommunity rt |
is-empty matches-any matches-every matches-within |
set additive delete-in delete-not-in delete-all |
|
|
extcommunity soo |
is-empty matches-any matches-every matches-within |
— | |
|
local-preference |
is, ge, le, eq |
set |
|
|
med |
is, eg, ge, le |
set set + set - set max-unreachable set igp-cost |
|
|
next-hop |
in |
set set self |
|
|
origin |
is |
set |
|
|
path-type |
is |
— | |
|
rd |
in |
— | |
|
route-aggregated |
route-aggregated |
— |
|
|
source |
in |
— | |
|
unsuppress-route |
— |
unsuppress-route |
|
|
vpn-distinguisher |
— |
set |
|
|
neighbor-orf |
orf-prefix |
in |
n/a |
|
network |
as-path |
— |
prepend |
|
community |
— |
set set additive delete-in delete-not-in delete-all |
|
|
destination |
in |
— | |
|
extcommunity cost |
— |
set set additive |
|
|
mpls-label |
route-has-label |
— | |
|
local-preference |
— |
set |
|
|
med |
— |
set set+ set- |
|
|
next-hop |
in |
set |
|
|
origin |
— |
set |
|
|
route-type |
is |
— | |
|
tag |
is, ge, le, eq |
— | |
|
weight |
— |
set |
|
|
next-hop |
destination |
in |
— |
|
protocol |
is,in |
— | |
|
source |
in |
— | |
|
redistribute |
as-path |
— |
prepend |
|
community |
— |
set set additive delete in delete not in delete all |
|
|
destination |
in |
— | |
|
extcommunity cost |
— |
setset additive |
|
|
local-preference |
— |
set |
|
|
med |
— |
set set+ set- |
|
|
next-hop |
in |
set |
|
|
origin |
— |
set |
|
|
mpls-label |
route-has-label |
— | |
|
route-type |
is |
— | |
|
tag |
is, eq, ge, le |
— | |
|
weight |
— |
set |
|
|
retain-rt |
extcommunity rt |
is-empty matches-any matches-every matches-within |
— |
|
show |
as-path |
in is-local length neighbor-is originates-from passes-through unique-length |
— |
|
as-path-length |
is, ge, le, eq |
— | |
|
as-path-unique-length |
is, ge, le, eq |
— | |
|
community |
is-empty matches-any matches-every |
— | |
|
destination |
in |
— | |
|
extcommunity rt |
is-empty matches-any matches-every matches-within |
— | |
|
extcommunity soo |
is-empty matches-any matches-every matches-within |
— | |
|
med |
is, eg, ge, le |
— | |
|
next-hop |
in |
— | |
|
origin |
is |
— | |
|
source |
in |
— |
This table summarizes which operations are valid and where they are valid.
|
Command |
import |
export |
aggregation |
redistribution |
|---|---|---|---|---|
|
prepend as-path most-recent |
eBGP only |
eBGP only |
n/a |
n/a |
|
replace as-path |
eBGP only |
eBGP only |
n/a |
n/a |
|
set med igp-cost |
forbidden |
eBGP only |
forbidden |
forbidden |
|
set weight |
n/a |
forbidden |
n/a |
n/a |
|
suppress |
forbidden |
forbidden |
n/a |
forbidden |
BGP Best Path Algorithm
BGP routers typically receive multiple paths to the same destination. The BGP best-path algorithm determines the best path to install in the IP routing table and to use for forwarding traffic. This section describes the Cisco IOS XR software implementation of BGP best-path algorithm, as specified in Section 9.1 of the Internet Engineering Task Force (IETF) Network Working Group draft-ietf-idr-bgp4-24.txt document.
The BGP best-path algorithm implementation is in three parts:
-
Part 1—Compares two paths to determine which is better.
-
Part 2—Iterates over all paths and determines which order to compare the paths to select the overall best path.
-
Part 3—Determines whether the old and new best paths differ enough so that the new best path should be used.
Note |
The order of comparison determined by Part 2 is important because the comparison operation is not transitive; that is, if three paths, A, B, and C exist, such that when A and B are compared, A is better, and when B and C are compared, B is better, it is not necessarily the case that when A and C are compared, A is better. This nontransitivity arises because the multi exit discriminator (MED) is compared only among paths from the same neighboring autonomous system (AS) and not among all paths. |
Comparing Pairs of Paths
Perform the following steps to compare two paths and determine the better path:
-
If either path is invalid (for example, a path has the maximum possible MED value or it has an unreachable next hop), then the other path is chosen (provided that the path is valid).
-
If the paths have unequal pre-bestpath cost communities, the path with the lower pre-bestpath cost community is selected as the best path.
-
If the paths have unequal weights, the path with the highest weight is chosen. Note
The weight is entirely local to the router, and can be set with the weight command or using a routing policy.
-
If the paths have unequal local preferences, the path with the higher local preference is chosen.
Note
If a local preference attribute was received with the path or was set by a routing policy, then that value is used in this comparison. Otherwise, the default local preference value of 100 is used. The default value can be changed using the bgp default local-preference command.
-
If one of the paths is a redistributed path, which results from a redistribute or network command, then it is chosen. Otherwise, if one of the paths is a locally generated aggregate, which results from an aggregate-address command, it is chosen.
Note
Step 1 through Step 4 implement the “Path Selection with BGP”of RFC 1268.
-
If the paths have unequal AS path lengths, the path with the shorter AS path is chosen. This step is skipped if bgp bestpath as-path ignore command is configured.
Note
When calculating the length of the AS path, confederation segments are ignored, and AS sets count as 1.
Note
eiBGP specifies internal and external BGP multipath peers. eiBGP allows simultaneous use of internal and external paths.
-
If the paths have different origins, the path with the lower origin is selected. Interior Gateway Protocol (IGP) is considered lower than EGP, which is considered lower than INCOMPLETE.
-
If appropriate, the MED of the paths is compared. If they are unequal, the path with the lower MED is chosen.
A number of configuration options exist that affect whether or not this step is performed. In general, the MED is compared if both paths were received from neighbors in the same AS; otherwise the MED comparison is skipped. However, this behavior is modified by certain configuration options, and there are also some corner cases to consider.
If the bgp bestpath med always command is configured, then the MED comparison is always performed, regardless of neighbor AS in the paths. Otherwise, MED comparison depends on the AS paths of the two paths being compared, as follows:
-
If a path has no AS path or the AS path starts with an AS_SET, then the path is considered to be internal, and the MED is compared with other internal paths.
-
If the AS path starts with an AS_SEQUENCE, then the neighbor AS is the first AS number in the sequence, and the MED is compared with other paths that have the same neighbor AS.
-
If the AS path contains only confederation segments or starts with confederation segments followed by an AS_SET, then the MED is not compared with any other path unless the bgp bestpath med confed command is configured. In that case, the path is considered internal and the MED is compared with other internal paths.
-
If the AS path starts with confederation segments followed by an AS_SEQUENCE, then the neighbor AS is the first AS number in the AS_SEQUENCE, and the MED is compared with other paths that have the same neighbor AS.
Note
If no MED attribute was received with the path, then the MED is considered to be 0 unless the bgp bestpath med missing-as-worst command is configured. In that case, if no MED attribute was received, the MED is considered to be the highest possible value.
-
-
If one path is received from an external peer and the other is received from an internal (or confederation) peer, the path from the external peer is chosen.
-
If the paths have different IGP metrics to their next hops, the path with the lower IGP metric is chosen.
-
If the paths have unequal IP cost communities, the path with the lower IP cost community is selected as the best path.
-
If all path parameters in Step 1 through Step 10 are the same, then the router IDs are compared. If the path was received with an originator attribute, then that is used as the router ID to compare; otherwise, the router ID of the neighbor from which the path was received is used. If the paths have different router IDs, the path with the lower router ID is chosen.
Note
Where the originator is used as the router ID, it is possible to have two paths with the same router ID. It is also possible to have two BGP sessions with the same peer router, and therefore receive two paths with the same router ID.
-
If the paths have different cluster lengths, the path with the shorter cluster length is selected. If a path was not received with a cluster list attribute, it is considered to have a cluster length of 0.
-
Finally, the path received from the neighbor with the lower IP address is chosen. Locally generated paths (for example, redistributed paths) are considered to have a neighbor IP address of 0.
Order of Comparisons
The second part of the BGP best-path algorithm implementation determines the order in which the paths should be compared. The order of comparison is determined as follows:
-
The paths are partitioned into groups such that within each group the MED can be compared among all paths. The same rules as in are used to determine whether MED can be compared between any two paths. Normally, this comparison results in one group for each neighbor AS. If the bgp bestpath med always command is configured, then there is just one group containing all the paths.
-
The best path in each group is determined. Determining the best path is achieved by iterating through all paths in the group and keeping track of the best one seen so far. Each path is compared with the best-so-far, and if it is better, it becomes the new best-so-far and is compared with the next path in the group.
-
A set of paths is formed containing the best path selected from each group in Step 2. The overall best path is selected from this set of paths, by iterating through them as in Step 2.
Best Path Change Suppression
The third part of the implementation is to determine whether the best-path change can be suppressed or not—whether the new best path should be used, or continue using the existing best path. The existing best path can continue to be used if the new one is identical to the point at which the best-path selection algorithm becomes arbitrary (if the router-id is the same). Continuing to use the existing best path can avoid churn in the network.
Note |
This suppression behavior does not comply with the IETF Networking Working Group draft-ietf-idr-bgp4-24.txt document, but is specified in the IETF Networking Working Group draft-ietf-idr-avoid-transition-00.txt document. |
The suppression behavior can be turned off by configuring the bgp bestpath compare-routerid command. If this command is configured, the new best path is always preferred to the existing one.
Otherwise, the following steps are used to determine whether the best-path change can be suppressed:
-
If the existing best path is no longer valid, the change cannot be suppressed.
-
If either the existing or new best paths were received from internal (or confederation) peers or were locally generated (for example, by redistribution), then the change cannot be suppressed. That is, suppression is possible only if both paths were received from external peers.
-
If the paths were received from the same peer (the paths would have the same router-id), the change cannot be suppressed. The router ID is calculated using rules in .
-
If the paths have different weights, local preferences, origins, or IGP metrics to their next hops, then the change cannot be suppressed. Note that all these values are calculated using the rules in .
-
If the paths have different-length AS paths and the bgp bestpath as-path ignore command is not configured, then the change cannot be suppressed. Again, the AS path length is calculated using the rules in .
-
If the MED of the paths can be compared and the MEDs are different, then the change cannot be suppressed. The decision as to whether the MEDs can be compared is exactly the same as the rules in , as is the calculation of the MED value.
-
If all path parameters in Step 1 through Step 6 do not apply, the change can be suppressed.
BGP Update Generation and Update Groups
The BGP Update Groups feature separates BGP update generation from neighbor configuration. The BGP Update Groups feature introduces an algorithm that dynamically calculates BGP update group membership based on outbound routing policies. This feature does not require any configuration by the network operator. Update group-based message generation occurs automatically and independently.
BGP Update Group
When a change to the configuration occurs, the router automatically recalculates update group memberships and applies the changes.
For the best optimization of BGP update group generation, we recommend that the network operator keeps outbound routing policy the same for neighbors that have similar outbound policies. This feature contains commands for monitoring BGP update groups.
BGP Cost Community Reference
The cost community attribute is applied to internal routes by configuring the set extcommunity cost command in a route policy. The cost community set clause is configured with a cost community ID number (0–255) and cost community number (0–4294967295). The cost community number determines the preference for the path. The path with the lowest cost community number is preferred. Paths that are not specifically configured with the cost community number are assigned a default cost community number of 2147483647 (the midpoint between 0 and 4294967295) and evaluated by the best-path selection process accordingly. When two paths have been configured with the same cost community number, the path selection process prefers the path with the lowest cost community ID. The cost-extended community attribute is propagated to iBGP peers when extended community exchange is enabled.
The following commands include the route-policy keyword, which you can use to apply a route policy that is configured with the cost community set clause:
-
aggregate-address
-
redistribute
-
network
BGP Next Hop Reference
-
Next hop becomes unreachable
-
Next hop becomes reachable
-
Fully recursed IGP metric to the next hop changes
-
First hop IP address or first hop interface change
-
Next hop becomes connected
-
Next hop becomes unconnected
-
Next hop becomes a local address
-
Next hop becomes a nonlocal address
Note |
Reachability and recursed metric events trigger a best-path recalculation. |
-
Critical events are related to the reachability (reachable and unreachable), connectivity (connected and unconnected), and locality (local and nonlocal) of the next hops. Notifications for these events are not delayed.
-
Noncritical events include only the IGP metric changes. These events are sent at an interval of 3 seconds. A metric change event is batched and sent 3 seconds after the last one was sent.
BGP is notified when any of the following events occurs:
-
Next hop becomes unreachable
-
Next hop becomes reachable
-
Fully recursed IGP metric to the next hop changes
-
First hop IP address or first hop interface change
-
Next hop becomes connected
-
Next hop becomes unconnected
-
Next hop becomes a local address
-
Next hop becomes a nonlocal address
Note |
Reachability and recursed metric events trigger a best-path recalculation. |
The next-hop trigger delay for critical and noncritical events can be configured to specify a minimum batching interval for critical and noncritical events using the nexthop trigger-delay command. The trigger delay is address family dependent.
The BGP next-hop tracking feature allows you to specify that BGP routes are resolved using only next hops whose routes have the following characteristics:
-
To avoid the aggregate routes, the prefix length must be greater than a specified value.
-
The source protocol must be from a selected list, ensuring that BGP routes are not used to resolve next hops that could lead to oscillation.
This route policy filtering is possible because RIB identifies the source protocol of route that resolved a next hop as well as the mask length associated with the route. The nexthop route-policy command is used to specify the route-policy.
Next Hop as the IPv6 Address of Peering Interface
BGP can carry IPv6 prefixes over an IPv4 session. The next hop for the IPv6 prefixes can be set through a nexthop policy. In the event that the policy is not configured, the nexthops are set as the IPv6 address of the peering interface (IPv6 neighbor interface or IPv6 update source interface, if any one of the interfaces is configured).
If the nexthop policy is not configured and neither the IPv6 neighbor interface nor the IPv6 update source interface is configured, the next hop is the IPv4 mapped IPv6 address.
Scoped IPv4/VPNv4 Table Walk
To determine which address family to process, a next-hop notification is received by first de-referencing the gateway context associated with the next hop, then looking into the gateway context to determine which address families are using the gateway context. The IPv4 unicast and VPNv4 unicast address families share the same gateway context, because they are registered with the IPv4 unicast table in the RIB. As a result, both the global IPv4 unicast table and the VPNv4 table are is processed when an IPv4 unicast next-hop notification is received from the RIB. A mask is maintained in the next hop, indicating if whether the next hop belongs to IPv4 unicast or VPNv4 unicast, or both. This scoped table walk localizes the processing in the appropriate address family table.
Reordered Address Family Processing
The software walks address family tables based on the numeric value of the address family. When a next-hop notification batch is received, the order of address family processing is reordered to the following order:
-
IPv4 tunnel
-
VPNv4 unicast
-
VPNv6 unicast
-
IPv4 labeled unicast
-
IPv4 unicast
-
IPv4 MDT
-
IPv6 unicast
-
IPv6 labeled unicast
-
IPv4 tunnel
-
VPNv4 unicast
-
IPv4 unicast
-
IPv6 unicast
New Thread for Next-Hop Processing
The critical-event thread in the spkr process handles only next-hop, Bidirectional Forwarding Detection (BFD), and fast-external-failover (FEF) notifications. This critical-event thread ensures that BGP convergence is not adversely impacted by other events that may take a significant amount of time.
show, clear, and debug Commands
The show bgp nexthops command provides statistical information about next-hop notifications, the amount of time spent in processing those notifications, and details about each next hop registered with the RIB. The clear bgp nexthop performance-statistics command ensures that the cumulative statistics associated with the processing part of the next-hop show command can be cleared to help in monitoring. The clear bgp nexthop registration command performs an asynchronous registration of the next hop with the RIB.
The debug bgp nexthop command displays information on next-hop processing. The out keyword provides debug information only about BGP registration of next hops with RIB. The in keyword displays debug information about next-hop notifications received from RIB. The out keyword displays debug information about next-hop notifications sent to the RIB.
BGP Nonstop Routing Reference
BGP NSR provides nonstop routing during the following events:
-
Route processor switchover
-
Process crash or process failure of BGP or TCP Note
BGP NSR is enabled by default. Use the nsr disable command to turn off BGP NSR. The no nsr disable command can also be used to turn BGP NSR back on if it has been disabled.
In case of process crash or process failure, NSR will be maintained only if nsr process-failures switchover command is configured. In the event of process failures of active instances, the nsr process-failures switchover configures failover as a recovery action and switches over to a standby route processor (RP) or a standby distributed route processor (DRP) thereby maintaining NSR. An example of the configuration command is RP/0/RSP0/CPU0:router(config) # nsr process-failures switchover
The nsr process-failures switchover command maintains both the NSR and BGP sessions in the event of a BGP or TCP process crash. Without this configuration, BGP neighbor sessions flap in case of a BGP or TCP process crash. This configuration does not help if the BGP or TCP process is restarted in which case the BGP neighbors are expected to flap.
When the l2vpn_mgr process is restarted, the NSR client (te-control) flaps between the Ready and Not Ready state. This is the expected behavior and there is no traffic loss.
During route processor switchover and In-Service System Upgrade (ISSU), NSR is achieved by stateful switchover (SSO) of both TCP and BGP.
NSR does not force any software upgrades on other routers in the network, and peer routers are not required to support NSR.
When a route processor switchover occurs due to a fault, the TCP connections and the BGP sessions are migrated transparently to the standby route processor, and the standby route processor becomes active. The existing protocol state is maintained on the standby route processor when it becomes active, and the protocol state does not need to be refreshed by peers.
Events such as soft reconfiguration and policy modifications can trigger the BGP internal state to change. To ensure state consistency between active and standby BGP processes during such events, the concept of post-it is introduced that act as synchronization points.
BGP NSR provides the following features:
-
NSR-related alarms and notifications
-
Configured and operational NSR states are tracked separately
-
NSR statistics collection
-
NSR statistics display using show commands
-
XML schema support
-
Auditing mechanisms to verify state synchronization between active and standby instances
-
CLI commands to enable and disable NSR
BGP Route Reflectors Reference
#concept_305B2C3244D5404F9E6207098D05CA9E__ illustrates a simple iBGP configuration with three iBGP speakers (routers A, B, and C). Without route reflectors, when Router A receives a route from an external neighbor, it must advertise it to both routers B and C. Routers B and C do not readvertise the iBGP learned route to other iBGP speakers because the routers do not pass on routes learned from internal neighbors to other internal neighbors, thus preventing a routing information loop.
With route reflectors, all iBGP speakers need not be fully meshed because there is a method to pass learned routes to neighbors. In this model, an iBGP peer is configured to be a route reflector responsible for passing iBGP learned routes to a set of iBGP neighbors. In #concept_305B2C3244D5404F9E6207098D05CA9E__ , Router B is configured as a route reflector. When the route reflector receives routes advertised from Router A, it advertises them to Router C, and vice versa. This scheme eliminates the need for the iBGP session between routers A and C.
The internal peers of the route reflector are divided into two groups: client peers and all other routers in the autonomous system (nonclient peers). A route reflector reflects routes between these two groups. The route reflector and its client peers form a cluster. The nonclient peers must be fully meshed with each other, but the client peers need not be fully meshed. The clients in the cluster do not communicate with iBGP speakers outside their cluster.
#concept_305B2C3244D5404F9E6207098D05CA9E__ illustrates a more complex route reflector scheme. Router A is the route reflector in a cluster with routers B, C, and D. Routers E, F, and G are fully meshed, nonclient routers.
When the route reflector receives an advertised route, depending on the neighbor, it takes the following actions:
-
A route from an external BGP speaker is advertised to all clients and nonclient peers.
-
A route from a nonclient peer is advertised to all clients.
-
A route from a client is advertised to all clients and nonclient peers. Hence, the clients need not be fully meshed.
Along with route reflector-aware BGP speakers, it is possible to have BGP speakers that do not understand the concept of route reflectors. They can be members of either client or nonclient groups, allowing an easy and gradual migration from the old BGP model to the route reflector model. Initially, you could create a single cluster with a route reflector and a few clients. All other iBGP speakers could be nonclient peers to the route reflector and then more clusters could be created gradually.
An autonomous system can have multiple route reflectors. A route reflector treats other route reflectors just like other iBGP speakers. A route reflector can be configured to have other route reflectors in a client group or nonclient group. In a simple configuration, the backbone could be divided into many clusters. Each route reflector would be configured with other route reflectors as nonclient peers (thus, all route reflectors are fully meshed). The clients are configured to maintain iBGP sessions with only the route reflector in their cluster.
Usually, a cluster of clients has a single route reflector. In that case, the cluster is identified by the router ID of the route reflector. To increase redundancy and avoid a single point of failure, a cluster might have more than one route reflector. In this case, all route reflectors in the cluster must be configured with the cluster ID so that a route reflector can recognize updates from route reflectors in the same cluster. All route reflectors serving a cluster should be fully meshed and all of them should have identical sets of client and nonclient peers.
By default, the clients of a route reflector are not required to be fully meshed and the routes from a client are reflected to other clients. However, if the clients are fully meshed, the route reflector need not reflect routes to clients.
As the iBGP learned routes are reflected, routing information may loop. The route reflector model has the following mechanisms to avoid routing loops:
-
Originator ID is an optional, nontransitive BGP attribute. It is a 4-byte attributed created by a route reflector. The attribute carries the router ID of the originator of the route in the local autonomous system. Therefore, if a misconfiguration causes routing information to come back to the originator, the information is ignored.
-
Cluster-list is an optional, nontransitive BGP attribute. It is a sequence of cluster IDs that the route has passed. When a route reflector reflects a route from its clients to nonclient peers, and vice versa, it appends the local cluster ID to the cluster-list. If the cluster-list is empty, a new cluster-list is created. Using this attribute, a route reflector can identify if routing information is looped back to the same cluster due to misconfiguration. If the local cluster ID is found in the cluster-list, the advertisement is ignored.
iBGP Multipath Load Sharing Reference
When there are multiple border BGP routers having reachability information heard over eBGP, if no local policy is applied, the border routers will choose their eBGP paths as best. They advertise that bestpath inside the ISP network. For a core router, there can be multiple paths to the same destination, but it will select only one path as best and use that path for forwarding. iBGP multipath load sharing adds the ability to enable load sharing among multiple equi-distant paths. Configuring multiple iBGP best paths enables a router to evenly share the traffic destined for a particular site. The iBGP Multipath Load Sharing feature functions similarly in a Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) with a service provider backbone.
For multiple paths to the same destination to be considered as multipaths, the following criteria must be met:
-
All attributes must be the same. The attributes include weight, local preference, autonomous system path (entire attribute and not just length), origin code, Multi Exit Discriminator (MED), and Interior Gateway Protocol (iGP) distance.
-
The next hop router for each multipath must be different.
Note |
|
L3VPN iBGP PE-CE Reference
When BGP is used as the provider edge (PE) or the customer edge (CE) routing protocol, the peering sessions are configured as external peering between the VPN provider autonomous system (AS) and the customer network autonomous system. The L3VPN iBGP PE-CE feature enables the PE and CE devices to exchange Border Gateway Protocol (BGP) routing information by peering as internal Border Gateway Protocol (iBGP) instead of the widely-used external BGP peering between the PE and the CE. This mechanism applies at each PE device where a VRF-based CE is configured as iBGP. This eliminates the need for service providers (SPs) to configure autonomous system override for the CE. With this feature enabled, there is no need to configure the virtual private network (VPN) sites using different autonomous systems.
The neighbor internal-vpn-client command enables PE devices to make an entire VPN cloud act as an internal VPN client to the CE devices. These CE devices are connected internally to the VPN cloud through the iBGP PE-CE connection inside the VRF. After this connection is established, the PE device encapsulates the CE-learned path into an attribute called ATTR_SET and carries it in the iBGP-sourced path throughout the VPN core to the remote PE device. At the remote PE device, this attribute is assigned with individual attributes and the source CE path is extracted and sent to the remote CE devices.
+------------------------------+
| Attr Flags (O|T) Code = 128 |
+------------------------------+
| Attr. Length (1 or 2 octets) |
+------------------------------+
| Origin AS (4 octets) |
+------------------------------+
| Path attributes (variable) |
+------------------------------+
Origin AS is the AS of the VPN customer for which the ATTR_SET is generated. The minimum length of ATTR_SET is four bytes and the maximum is the maximum supported for a path attribute after taking into consideration the mandatory fields and attributes in the BGP update message. It is recommended that the maximum length is limited to 3500 bytes. ATTR_SET must not contain the following attributes: MP_REACH, MP_UNREACH, NEW_AS_PATH, NEW_AGGR, NEXT_HOP and ATTR_SET itself (ATTR_SET inside ATTR_SET). If these attributes are found inside the ATTR_SET, the ATTR_SET is considered invalid and the corresponding error handling mechanism is invoked.
Per VRF and Per CE Label for IPv6 Provider Edge
The per VRF and per CE label for IPv6 feature makes it possible to save label space by allocating labels per default VRF or per CE nexthop.
All IPv6 Provider Edge (6PE) labels are allocated per prefix by default. Each prefix that belongs to a VRF instance is advertised with a single label, causing an additional lookup to be performed in the VRF forwarding table to determine the customer edge (CE) next hop for the packet.
However, use the label-allocation-mode command with the per-ce keyword or the per-vrf keyword to avoid the additional lookup on the PE router and conserve label space.
Use per-ce keyword to specify that the same label be used for all the routes advertised from a unique customer edge (CE) peer router. Use the per-vrf keyword to specify that the same label be used for all the routes advertised from a unique VRF.
IPv6 Unicast Routing
Cisco provides complete Internet Protocol Version 6 (IPv6) unicast capability.
An IPv6 unicast address is an identifier for a single interface, on a single node. A packet that is sent to a unicast address is delivered to the interface identified by that address. Cisco IOS XR software supports the following IPv6 unicast address types:
-
Global aggregatable address
-
Site-local address
-
Link-local address
-
IPv4-compatible IPv6 address
For more information on IPv6 unicast addressing, refer the IP Addresses and Services Configuration Guide.
Remove and Replace Private AS Numbers from AS Path in BGP
Private autonomous system numbers (ASNs) are used by Internet Service Providers (ISPs) and customer networks to conserve globally unique AS numbers. Private AS numbers cannot be used to access the global Internet because they are not unique. AS numbers appear in eBGP AS paths in routing updates. Removing private ASNs from the AS path is necessary if you have been using private ASNs and you want to access the global Internet.
Public AS numbers are assigned by InterNIC and are globally unique. They range from 1 to 64511. Private AS numbers are used to conserve globally unique AS numbers, and they range from 64512 to 65535. Private AS numbers cannot be leaked to a global BGP routing table because they are not unique, and BGP best path calculations require unique AS numbers. Therefore, it might be necessary to remove private AS numbers from an AS path before the routes are propagated to a BGP peer.
External BGP (eBGP) requires that globally unique AS numbers be used when routing to the global Internet. Using private AS numbers (which are not unique) would prevent access to the global Internet. The remove and replace private AS Numbers from AS Path in BGP feature allows routers that belong to a private AS to access the global Internet. A network administrator configures the routers to remove private AS numbers from the AS path contained in outgoing update messages and optionally, to replace those numbers with the ASN of the local router, so that the AS Path length remains unchanged.
The ability to remove and replace private AS numbers from the AS Path is implemented in the following ways:
-
The remove-private-as command removes private AS numbers from the AS path even if the path contains both public and private ASNs.
-
The remove-private-as command removes private AS numbers even if the AS path contains only private AS numbers. There is no likelihood of a 0-length AS path because this command can be applied to eBGP peers only, in which case the AS number of the local router is appended to the AS path.
-
The remove-private-as command removes private AS numbers even if the private ASNs appear before the confederation segments in the AS path.
-
The replace-as command replaces the private AS numbers being removed from the path with the local AS number, thereby retaining the same AS path length.
The feature can be applied to neighbors per address family (address family configuration mode). Therefore, you can apply the feature for a neighbor in one address family and not on another, affecting update messages on the outbound side for only the address family for which the feature is configured.
Use show bgp neighbors and show bgp update-group commands to verify that the that private AS numbers were removed or replaced.
Replace BGP AS Path with Custom Values
|
Feature Name |
Release Information |
Feature Description |
|---|---|---|
|
Replace BGP AS Path with Custom Values |
Release 7.5.2 |
You can now configure route policies to replace the Autonomous System (AS) Path in BGP with custom values to control the best path selection process. This feature introduces the replace as-path all command. |
BGP routers typically receive multiple paths to the same destination. The BGP best-path algorithm determines the best path to install in the IP routing table and to use for forwarding traffic. The overall best path is selected based on various attributes. .
AS path is one of the attributes used for best path selection. By default, BGP always prefers the route with shortest AS path as the best path. The best path selected by BGP might have traffic engineering issues, like heavy traffic that leads to congestion. In such cases, you can alter the best path by replacing the AS path with custom values.
The following are the custom values you can use to replace the AS path:
-
None: Use this option to modify an AS path as the shortest path in the network. When you choose this option, the AS path is replaced with a null or empty value. Use the replace as-path all none command to replace with none.
-
Auto: Use this option to advertise the local AS number or the neigbor's AS number as the AS path. When you choose this option, AS path is replaced based on the route policy:
-
For inbound route policy, AS path is replaced with AS path of BGP neighbor from where the prefix is received.
-
For outbound route policy, AS path is replaced with the local AS number.
Use the replace as-path all auto command to replace with auto.
-
-
'x': Use this option to replace AS path with any specified value. Use the replace as-path all 'x' command to replace with this option, where 'x' can be a single AS number or a sequence of AS numbers separated by space.
-
Optionally, you can repeat replacing the AS path for a specified number of times. This option is supported only for the auto and 'x' parameters. Use the replace as-path all {auto | 'x'} [n] command to enable the repeat option.
-
Optionally, you can use a parameter name along with the repeat option. The parameter name must be preceded with a “$.” You can attach the route policy with the parameter to a neighbor and specify the number of times the AS path replacement should be repeated. This opton allows you to apply the same route policy to different neighbors with different AS path values.
Use the replace as-path all {auto | 'x'} [n] [parameter] command to enable the parameter along with repeat option.
You can replace the AS path for inbound eBGP, outbound eBGP, and outbound iBGP paths.
Note |
For outbound eBGP paths, the AS number of the local router is always prepended to the replaced AS path. |
Interoperability with BGP Confederation
BGP confederation is a group of multiple autonomous systems that looks like a single autonomous system to the outside world. When confederation is configured on BGP peers, the AS path is replaced as follows:
-
When you replace the AS path in an outbound BGP router, which receives prefix from a BGP neighbor configured with confederation, the specified AS path value is appended to the confederation sequence.
-
When you replace the AS path in an inbound BGP router configured with confederation, the confederation sequence is replaced with the specified AS path value.
Deployment Scenario
Consider a BGP network configured with AS paths. By default, BGP selects the route with shortest AS path to reach the destination. You can alter the default route by using the replace BGP AS path feature.
In the following figure, the network consists of BGP routers configured with AS Path values. To reach Server B, Server A typically selects Path B (via S1_1, S0), as the AS path value of S0 is shorter.
You may want to use Path A to reach the destination (via S1_1, S2_1, S1_2, S0), for traffic engineering purpose. For example, Path A may be less congested and is better than Path B. To use Path A, you can replace the AS path values with one of the following options:
-
Replace AS path of Router S2_1 with a shorter value.
-
Replace AS path of Router S0 with a longer value.
Restrictions
-
The replace as-path all command isn't supported on inbound iBGP paths.
-
The replace as-path all command isn't supported on a route policy that is already configured with remove-private-as or replace as commands.
-
You can apply the route policy configured with replace as-path all only on neighbor-in or neighbor-out attach points.
Configuration Example
To replace BGP AS path with custom values, perform the following tasks on a BGP router:
This example shows how to replace AS path with null value.
/*Configure route policy to replace AS path with none*/
Router(config)#hw-module profile stats ?
Router(config)# route-policy aspath-none
Router(config-rpl)# replace as-path all none
Router(config-rpl)# end-policy
/* Apply route policy to BGP neighbor */
Router(config)# router bgp 65530
Router(config-bgp)# neighbor 111.0.0.1
Router(config-bgp-nbr)# address-family ipv4 unicast
Router(config-bgp-nbr-af)# route-policy aspath-none inThis example shows how to replace AS path with auto option.
/*Configure route policy to replace AS path with auto*/
Router(config)#route-policy aspath-auto
Router(config-rpl)# replace as-path all auto
Router(config-rpl)# end-policy
/* Apply route policy to BGP neighbor */
Router(config)# router bgp 65530
Router(config-bgp)# neighbor 111.0.0.1
Router(config-bgp-nbr)# address-family ipv4 unicast
Router(config-bgp-nbr-af)# route-policy aspath-auto out
This example shows how to replace AS path with a specified sequence of AS numbers. In this example, sequence '10 100 200 300' is used.
/*Configure route policy to replace AS path with 'x'*/
Router(config)# route-policy aspath-str
Router(config-rpl)# replace as-path all '10 100 200 300'
Router(config-rpl)# end-policy
/* Apply route policy to BGP neighbor */
Router(config)# router bgp 1
Router(config-bgp)# neighbor 111.0.0.1
Router(config-bgp-nbr)# address-family ipv4 unicast
Router(config-bgp-nbr-af)# route-policy aspath-str in
This example shows how to use replace as-path all command along with parameter to replace the AS path with specified sequence of values, repeated for specified number of times. In this example, AS path is replaced with sequence '45 55', repeated for 6 times.
/*Configure route policy to replace AS path with parameter ($n)*/
Router(config)# route-policy aspath-par($n)
Router(config-rpl)# replace as-path all '45 55' $n
Router(config-rpl)# end-policy
/* Apply route policy to BGP neighbor */
Router(config)# router bgp 1
Router(config-bgp)# neighbor 111.0.0.1
Router(config-bgp-nbr)# address-family ipv4 unicast
Router(config-bgp-nbr-af)# route-policy aspath-par(6) in
Verification
In the following output, AS path is replaced with null value.
Router# show bgp
Network Next Hop Metric LocPrf Weight Path
*> 192.168.3.0/24 192.168.3.1 0 0 iIn the following output, AS path is replaced with auto for an outbound path, where the AS path of local router is [40].
Router# show bgp
Network Next Hop Metric LocPrf Weight Path
*> 111.0.0.2/32 200.0.0.5 0 40 i
In the following output, AS path is replaced with the sequence '10 100 200 300'.
Router# show bgp
Network Next Hop Metric LocPrf Weight Path
*>111.0.0.2/32 200.0.0.5 0 10 100 200 300 iIn the following output, AS path is replaced with the sequence '45 55', repeated for 6 times.
Router# show bgp
Network Next Hop Metric LocPrf Weight Path
*>111.0.0.8/32 200.0.0.5 0 45 55 45 55 45 55 45 55 45 55 45 55 i
Configure Replace BGP AS Path with Custom Values
Perform the following steps to replace BGP AS path with custom values.
Procedure
|
Step 1 |
configure Example:Enters global configuration mode. |
||
|
Step 2 |
route-policy route-policy-name Example:Defines the route policy and enters route-policy configuration mode. |
||
|
Step 3 |
replace as-path all { none | auto | x } [n ] [parameter ] Replaces the entire AS path with specified values.
Example: |
||
|
Step 4 |
Use the show bgp command to verify the replaced AS path. Example: |
BGP Update Message Error Handling
The BGP UPDATE message error handling changes BGP behavior in handling error UPDATE messages to avoid session reset. Based on the approach described in IETF IDR I-D:draft-ietf-idr-error-handling, the Cisco IOS XR BGP UPDATE Message Error handling implementation classifies BGP update errors into various categories based on factors such as, severity, likelihood of occurrence of UPDATE errors, or type of attributes. Errors encountered in each category are handled according to the draft. Session reset will be avoided as much as possible during the error handling process. Error handling for some of the categories are controlled by configuration commands to enable or disable the default behavior.
According to the base BGP specification, a BGP speaker that receives an UPDATE message containing a malformed attribute is required to reset the session over which the offending attribute was received. This behavior is undesirable as a session reset would impact not only routes with the offending attribute, but also other valid routes exchanged over the session.
BGP Error Handling and Attribute Filtering Syslog Messages
When a router receives a malformed update packet, an ios_msg of type ROUTING-BGP-3-MALFORM_UPDATE is printed on the console. This is rate limited to 1 message per minute across all neighbors. For malformed packets that result in actions "Discard Attribute" (A5) or "Local Repair" (A6), the ios_msg is printed only once per neighbor per action. This is irrespective of the number of malformed updates received since the neighbor last reached an "Established" state.
%ROUTING-BGP-3-MALFORM_UPDATE : Malformed UPDATE message received from neighbor 13.0.3.50 - message length 90 bytes,
error flags 0x00000840, action taken "TreatAsWithdraw".
Error details: "Error 0x00000800, Field "Attr-missing", Attribute 1 (Flags 0x00, Length 0), Data []"
[4843.46]RP/0/RP0/CPU0:Aug 21 17:06:17.919 : bgp[1037]: %ROUTING-BGP-5-UPDATE_FILTERED :
One or more attributes were filtered from UPDATE message received from neighbor 40.0.101.1 - message length 173 bytes,
action taken "DiscardAttr".
Filtering details: "Attribute 16 (Flags 0xc0): Action "DiscardAttr"". NLRIs: [IPv4 Unicast] 88.2.0.0/17
[391.01]RP/0/RP0/CPU0:Aug 20 19:41:29.243 : bgp[1037]: %ROUTING-BGP-5-UPDATE_FILTERED :
One or more attributes were filtered from UPDATE message received from neighbor 40.0.101.1 - message length 166 bytes,
action taken "TreatAsWdr".
Filtering details: "Attribute 4 (Flags 0xc0): Action "TreatAsWdr"". NLRIs: [IPv4 Unicast] 88.2.0.0/17
BGP-RIB Feedback Mechanism for Update Generation
The Border Gateway Protocol-Routing Information Base (BGP-RIB) feedback mechanism for update generation feature avoids premature route advertisements and subsequent packet loss in a network. This mechanism ensures that routes are installed locally, before they are advertised to a neighbor.
BGP waits for feedback from RIB indicating that the routes that BGP installed in RIB are installed in forwarding information base (FIB) before BGP sends out updates to the neighbors. RIB uses the the BCDL feedback mechanism to determine which version of the routes have been consumed by FIB, and updates the BGP with that version. BGP will send out updates of only those routes that have versions up to the version that FIB has installed. This selective update ensures that BGP does not send out premature updates resulting in attracting traffic even before the data plane is programmed after router reload, LC OIR, or flap of a link where an alternate path is made available.
To configure BGP to wait for feedback from RIB indicating that the routes that BGP installed in RIB are installed in FIB, before BGP sends out updates to neighbors, use the update wait-install command in router address-family IPv4 or router address-family VPNv4 configuration mode. The show bgp , show bgp neighbors , and show bgp process performance-statistics commands display the information from update wait-install configuration.
Use-defined Martian Check
The solution allows disabling the Martian check for these IP address prefixes:
-
IPv4 address prefixes
-
0.0.0.0/8
-
127.0.0.0/8
-
224.0.0.0/4
-
-
IPv6 address prefixes
-
::
-
::0002 - ::ffff
-
::ffff:a.b.c.d
-
fe80:xxxx
-
ffxx:xxxx
-
Feedback