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	Other than enabling RTC (route target constraint) with address-family ipv4 rtfilter command, there is no separate configuration needed to enable RTC for BGP EVPN.

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).

Cisco IOS XR BGP imposes maximum limits on the number of neighbors that can be configured on the router and on the maximum number of prefixes that are accepted from a peer for a given address family. This limitation safeguards the router from resource depletion caused by misconfiguration, either locally or on the remote neighbor. The following limits apply to BGP configurations:

• The default maximum number of peers that can be configured is 4000. The default can be changed using the bgp maximum neighbor command. The limit range is 1 to 15000. Any attempt to configure additional peers beyond the maximum limit or set the maximum limit to a number that is less than the number of peers currently configured will fail.

• To prevent a peer from flooding BGP with advertisements, a limit is placed on the number of prefixes that are accepted from a peer for each supported address family. The default limits can be overridden through configuration of the maximum-prefix limit command for the peer for the appropriate address family. The following default limits are used if the user does not configure the maximum number of prefixes for the address family:

  • IPv4 Unicast: 1048576

  • IPv4 Labeled-unicast: 131072

  • IPv6 Unicast: 524288

  • IPv6 Labeled-unicast: 131072

  A cease notification message is sent to the neighbor and the peering with the neighbor is terminated when the number of prefixes received from the peer for a given address family exceeds the maximum limit (either set by default or configured by the user) for that address family.

  It is possible that the maximum number of prefixes for a neighbor for a given address family has been configured after the peering with the neighbor has been established and a certain number of prefixes have already been received from the neighbor for that address family. A cease notification message is sent to the neighbor and peering with the neighbor is terminated immediately after the configuration if the configured maximum number of prefixes is fewer than the number of prefixes that have already been received from the neighbor for the address family.

The BGP multipath selection algorithm functionality enables the multipath to prefer the older paths over the new paths. Here the age order is a vital criterion for selection of the UCMP. However, this method is less optimal and is nondeterministic in terms of forwarding traffic on the network. The BGP Enhanced Multipath Selection feature allows the multipath functionality to select IGP metric.

Router(config)# router bgp 100
Router(config-bgp)# address-family ipv4 unicast
Router(config-bgp-af)# maximum-paths ibgp 2 unequal-cost deterministic

router bgp 100
 address-family ipv4 unicast
  maximum-paths ibgp 2 unequal-cost deterministic

Router# show bgp ipv4 unicast 10.10.0.0/28
Paths: (128 available, best #1)
  Not advertised to any peer
  Path #1: Received by speaker 0
  Not advertised to any peer
  Local
    22.0.1.6 (metric 20) from 198.51.100.1 (192.0.0.1)
      Origin IGP, localpref 0, valid, internal, best, group-best, multipath
      Received Path ID 1, Local Path ID 1, version 12611
      Originator: 192.0.0.1, Cluster list: 198.51.100.1
…

Path #64: Received by speaker 0
  Not advertised to any peer
  Local
    23.0.11.6 (metric 30) from 203.0.113.1 (210.0.0.10)
      Origin IGP, localpref 0, valid, internal, multipath
      Received Path ID 32, Local Path ID 0, version 0
      Originator: 210.0.0.10, Cluster list: 203.0.113.1, 200.0.0.10

Path #65: Received by speaker 0
  Not advertised to any peer
  Local
    24.0.1.6 (metric 40) from 192.0.2.254 (211.0.0.0)
      Origin IGP, localpref 0, valid, internal
      Received Path ID 1, Local Path ID 0, version 0
      Originator: 211.0.0.0, Cluster list: 192.0.2.254, 201.0.0.0, 202.0.0.0
…

  Path #128: Received by speaker 0
  Not advertised to any peer
  Local
    25.0.23.6 (metric 50) from 198.51.100.233 (195.0.0.23)
      Origin IGP, localpref 0, valid, internal
      Received Path ID 32, Local Path ID 0, version 0
      Originator: 195.0.0.23, Cluster list: 198.51.99.255

Router# show route ipv4 200.0.0.0/28
Routing entry for 200.0.0.0/28
 Known via "bgp 1", distance 200, metric 0, type internal
 Installed Oct 17 04:06:41.027 for 00:01:22
 outing Descriptor Blocks
    10.0.1.6, from 198.51.100.1, BGP multi path
      Route metric is 0
    10.0.2.6, from 198.51.100.1, BGP multi path
      Route metric is 0
    …    
    10.0.32.6, from 198.51.100.1, BGP multi path
      Route metric is 0
    198.51.100.253, from 203.0.113.1, BGP multi path
      Route metric is 0
    …
    198.51.100.252, from 203.0.113.1, BGP multi path
      Route metric is 0
  No advertising protos.

Router# show cef ipv4 200.0.0.0/28 detail
Level 1 - Load distribution: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
 [0] via 10.0.1.6/32, recursive
 [1] via 10.0.2.6/32, recursive

 [62] via 203.0.113.211/32, recursive
 [63] via 203.0.112.211/32, recursive

  via 10.0.1.6/32, 257 dependencies, recursive, bgp-multipath [flags 0x6080]
  path-idx 0 NHID 0x0 [0x7a6cdf90 0x0]
  next hop 22.0.1.6/32 via 22.0.0.0/8

  Load distribution: 0 (refcount 1)
  Hash  OK  Interface              Address
  0     Y   TenGigE0/1/0/0/7       remote

 via 203.0.113.211/32, 257 dependencies, recursive, bgp-multipath [flags 0x6080]
 path-idx 62 NHID 0x0 [0x7a6ce058 0x0]
  next hop 203.0.112.211/32 via 203.0.0.0/8

  Load distribution: 0 (refcount 1)

  Hash  OK  Interface                 Address
  2    Y   TenGigE0/1/0/0/4          remote

  via 203.0.32.6/32, 257 dependencies, recursive, bgp-multipath [flags 0x6080]
  path-idx 63 NHID 0x0 [0x7a6ce058 0x0]
  next hop 203.0.32.6/32 via 203.0.0.0/8

  Load distribution: 0 (refcount 1)

  Hash  OK  Interface                 Address
  63    Y   TenGigE0/1/0/0/4          remote

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:

• Determine whether a next hop is reachable.

• Find the fully recursed IGP metric to the next hop (used in the best-path calculation).

• Validate the received next hops.

• Calculate the outgoing next hops.

• Verify the reachability and connectedness of neighbors.

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.

Event notifications from the RIB are classified as critical and noncritical. Notifications for critical and noncritical events are sent in separate batches. However, a noncritical event is sent along with the critical events if the noncritical event is pending and there is a request to read the critical events.

• 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.

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.

For information on route policy filtering for next hops using the next-hop attach point, see the Implementing Routing Policy Language on Cisco IOS XR Software module of Cisco IOS XR Routing Configuration Guide (this publication).

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.

BGP in Cisco IOS XR software follows a neighbor-based configuration model that requires that all configurations for a particular neighbor be grouped in one place under the neighbor configuration. Peer groups are not supported for either sharing configuration between neighbors or for sharing update messages. The concept of peer group has been replaced by a set of configuration groups to be used as templates in BGP configuration and automatically generated update groups to share update messages between neighbors.

BGP does not support the concept of a default address family. An address family must be explicitly configured under the BGP router configuration for the address family to be activated in BGP. Similarly, an address family must be explicitly configured under a neighbor for the BGP session to be activated under that address family. It is not required to have any address family configured under the BGP router configuration level for a neighbor to be configured. However, it is a requirement to have an address family configured at the BGP router configuration level for the address family to be configured under a neighbor.

For default VRF, starting from Cisco IOS XR Software Release 6.2.x, both IPv4 Unicast and IPv4 Labeled-unicast address families are supported under the same neighbor.

For non-default VRF, both IPv4 Unicast and IPv4 Labeled-unicast address families are not supported under the same neighbor. However, the configuration is accepted on the Router with the following error:

bgp[1051]: %ROUTING-BGP-4-INCOMPATIBLE_AFI : IPv4 Unicast and IPv4 Labeled-unicast Address families together are not supported under the same neighbor.

When one BGP session has both IPv4 unicast and IPv4 labeled-unicast AFI/SAF, then the routing behavior is nondeterministic. Therefore, the prefixes may not be correctly advertised. Incorrect prefix advertisement results in reachability issues. In order to avoid such reachability issues, you must explicitly configure a route policy to advertise prefixes either through IPv4 unicast or through IPv4 labeled-unicast address families.

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.

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
  

  

  

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.

BGP policy accounting uses traffic indices that are set on BGP routes to track various counters. See the Implementing Routing Policy on Cisco IOS XR Software module in the Routing Configuration Guide for Cisco NCS 6000 Series Routers for details on table policy use. See the Cisco Express Forwarding Commands on Cisco IOS XR Software module in the IP Addresses and Services Command Reference for Cisco NCS 6000 Series Routers for details on BGP policy accounting.

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 ‘black hole’ where BGP advertises routes to neighbors that BGP does not install in its global routing table and forwarding table.

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.

To use this feature, you must understand the following concepts:

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.

The cost community attribute is applied to internal routes by configuring the set extcommunity cost command in a route policy. See the Routing Policy Language Commands on Cisco IOS XR Software module of Cisco IOS XR Routing Command Reference for information on the set extcommunity cost command. 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 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.

An administrative distance is a rating of the trustworthiness of a routing information source. In general, the higher the value, the lower the trust rating. For information on specifying the administrative distance for BGP, see the BGP Commands module of the Routing Command Reference for Cisco NCS 6000 Series Routers

Normally, a route can be learned through more than one protocol. Administrative distance is used to discriminate between routes learned from more than one protocol. The route with the lowest administrative distance is installed in the IP routing table. By default, BGP uses the administrative distances shown in Table 1.

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.

In most cases, when a route is learned through eBGP, it is installed in the IP routing table because of its distance (20). 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. See Back Door Example .

In Back Door Example , 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.

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.

One way to reduce the iBGP mesh is to divide an autonomous system into multiple subautonomous 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.

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.

Three Fully Meshed iBGP Speakers 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 Simple BGP Model with a Route Reflector , 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.

More Complex BGP Route Reflector Model 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.

Remotely triggered black hole (RTBH) filtering is a technique that provides the ability to drop undesirable traffic before it enters a protected network. RTBH filtering provides a method for quickly dropping undesirable traffic at the edge of the network, based on either source addresses or destination addresses by forwarding it to a null0 interface. RTBH filtering based on a destination address is commonly known as Destination-based RTBH filtering. Whereas, RTBH filtering based on a source address is known as Source-based RTBH filtering.

RTBH filtering is one of the many techniques in the security toolkit that can be used together to enhance network security in the following ways:

• Effectively mitigate DDoS and worm attacks

• Quarantine all traffic destined for the target under attack

• Enforce blocklist filtering

Note	RTBH is not supported in cases such as L3VPN iBGP route over NULL0.

Note	On Jericho2 TCAM-based platforms, when you configure a NULL interface, both destination-based RTBH filtering (D-RTBH) and source-based RTBH filtering (S-RTBH) are triggered.

RTBH is implemented by defining a route policy (RPL) to discard undesirable traffic at next-hop using set next-hop discard command.

RTBH filtering sets the next-hop of the victim's prefix to the null interface. The traffic destined to the victim is dropped at the ingress.

The set next-hop discard configuration is used in the neighbor inbound policy. When this config is applied to a path, though the primary next-hop is associated with the actual path but the RIB is updated with next-hop set to Null0. Even if the primary received next-hop is unreachable, the RTBH path is considered reachable and will be a candidate in the bestpath selection process. The RTBH path is readvertised to other peers with either the received next-hop or nexthop-self based on normal BGP advertisement rules.

A typical deployment scenario for RTBH filtering would require running internal Border Gateway Protocol (iBGP) at the access and aggregation points and configuring a separate device in the network operations center (NOC) to act as a trigger. The triggering device sends iBGP updates to the edge, that cause undesirable traffic to be forwarded to a null0 interface and dropped.

Consider below topology, where a rogue router is sending traffic to a border router.

route-policy RTBH-trigger
  if tag is 777 then
    set community (1234:4321, no-export) additive
    pass
  else
    pass
  endif
  end-policy

router bgp 65001
 address-family ipv4 unicast
  redistribute static route-policy RTBH-trigger
 !
 neighbor 192.168.102.1 
  remote-as 65001
  address-family ipv4 unicast
   route-policy bgp_all in
   route-policy bgp_all out

router static
 address-family ipv4 unicast
 10.7.7.7/32 Null0 tag 777

route-policy RTBH
  if community matches-any (1234:4321) then
    set next-hop discard
  else
    pass
  endif
end-policy

router bgp 65001
 address-family ipv4 unicast
 !
 neighbor 192.168.102.2 
  remote-as 65001
  address-family ipv4 unicast
   route-policy RTBH in
   route-policy bgp_all out

Most of the show commands provide address family (AFI) and subaddress family (SAFI) arguments (see RFC 1700 and RFC 2858 for information on AFI and SAFI). The Cisco IOS XR software parser provides the ability to set the afi and safi so that it is not necessary to specify them while running a show command. The parser commands are:

• set default-afi { ipv4 | ipv6 | all }

• set default-safi

The parser automatically sets the default afi value to ipv4 and default safi value to unicast . It is necessary to use only the parser commands to change the default afi value from ipv4 or default safi value from unicast . Any afi or safi keyword specified in a show command overrides the values set using the parser commands.

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.

See the System Security Configuration Guide for Cisco NCS 6000 Series Routers for information on keychain management.

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).

BGP Session Authentication and Integrity using TCP Authentication Option feature enables you to use stronger Message Authentication Codes that protect against replays, even for long-lived TCP connections. This feature also provides more details on the association of security with TCP connections than TCP MD5 Signature option (TCP MD5).

This feature supports the following functionalities of TCP MD5:

• Protection of long-lived connections such as BGP and LDP.

• Support for larger set of MACs with minimal changes to the system and operations

BGP Session Authentication and Integrity using TCP Authentication Option feature supports IPv6. It supports these two cryptographic algorithms: HMAC-SHA-1-96 and AES-128-CMAC-96.

You can use two sets of keys, namely Master Key Tuples and traffic keys to authenticate incoming and outgoing segments.

This feature applies different option identifier than TCP MD5. This feature cannot be used simultaneously with TCP MD5.

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 NSR provides nonstop routing during the following events:

• Route processor switchover

• Process crash or process failure of BGP or TCP

  Note

  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.

• Minimum Disruption Restart (MDR)

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

NSR can be provisioned on a multishelf router. The following guidelines should be observed when provisioning NSR on a multishelf router:

• When provisioning NSR for line cards installed on a single rack, provision the active and standby applications on the distributed route processor (DRP) of that rack. If a rack failure occurs, sessions are dropped, because all line cards go down.

• When provisioning NSR for line cards installed on different racks, use one of the following three options:

  • Provision the active and standby applications on a distributed route processor (DRP) redundant pair, where there is a separate route processor in each rack. This configuration uses up two revenue-producing line-card slots on each rack, but is the most secure configuration.

  • Provision the active and standby applications on a distributed route processor (DRP) pair that spans two racks. In this configuration, the active/standby role of the line cards is not dependent on the active/standby role of the DRPs. This is called flexible process redundancy and provides for rack loss and efficient use of LC slots. Use of distributed BGP is not required with this solution.

  Note

  Sessions on line cards in a lost rack are not protected with any of the above options, because there is no line-card redundancy. These options ensure only that sessions on other racks are not affected by a lost rack. However, lost sessions from a lost rack may cause some traffic loss on other racks, because destinations learned through those lost sessions may no longer have alternate routes. Also, rack loss may cause the CPUs on route processors of active racks to slow as they attempt to define new paths for some routes.

The Border Gateway Protocol (BGP) 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:

1. Determine the best path from the entire set of paths available for a prefix.

2. Eliminate the current best path.

3. Eliminate all the internal paths for the prefix.

4. From the remaining paths, eliminate all the paths that have the same next hop as that of the current best path.

5. 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

The advertise best-external command enables the advertisement of the best–external path in global address family configuration mode.

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.

The retain local-label command enables the retention of the local label until the network is converged.

Cisco IOS XR software provides the capability to run Border Gateway Protocol (BGP) over Generic Routing Encapsulation (GRE) tunnel interfaces.

GRE protocol transports packets of one protocol over another protocol by means of encapsulation. Service Providers can provide IP services between their networks that are connected together by a public network using GRE encapsulation to carry data securely over the public network.

The packet that needs to be transported is first encapsulated in a GRE header, which is further encapsulated in another protocol like IPv4 or IPv6 and then forwarded to the destination.

The Cisco IOS XR software GRE implementation is compliant with GRE encapsulation defined in RFC 2784. Key and Sequence numbering as defined in RFC 2890 is not supported in Cisco IOS XR software GRE implementation. To be backward compliant with RFC 1701, Cisco IOS XR software transmits GRE packets with Reserved0 field set to zero. A receiver that is compliant with RFC 1701 treats key present, sequence number, and strict source route as zero and do not expect key and sequence number. The Cisco IOS XR software discards a GRE packet with any of the bits in Reserved0 field set.

TheBorder Gateway Protocol (BGP) commands use disable keyword to disable a feature. The keyword inheritance-disable disables the inheritance of the feature properties from the parent level.

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.

Note	BGP Additional Path feature is not supported under vrf.

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.

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     

Traditional BGP multipath feature allows a router receiving parallel paths to the same destination to install the multiple paths in the routing table. By default, this multipath feature is applied to all configured peers. BGP selective multipath allows application of the multipath feature only to selected peers.

The BGP router receiving multiple paths is configured with the maximum-paths ... selective option. The iBGP/eBGP neighbors sharing multiple paths are configured with the multipath option, while being added as neighbors on the BGP router.

The following behavior is to be noted while using BGP selective multipath:

• BGP selective multipath does not impact best path calculations. A best path is always included in the set of multipaths.

• For VPN prefixes, the PE paths are always eligible to be multipaths.

For information on the maximum-paths and multipath commands, see the Cisco ASR 9000 Series Aggregation Services Router Routing Command Reference.

Topology

A sample topology to illustrate the configuration used in this section is shown in the following figure.

Router R4 receives parallel paths from Routers R1, R2 and R3 to the same destination. If Routers R1 and R2 are configured as selective multipath neighbors on Router R4, only the parallel paths from these routers are installed in the routing table of Router R4.

Configuration

Note	Configure your network topology with iBGP/eBGP running on your routers, before configuring this feature.

To configure BGP selective multipath on Router R4, use the following steps.

1. Configure Router R4 to accept selective multiple paths in your topology.

   
   /* To configure selective multipath for iBGP/eBGP
   RP/0/RP0/CPU0:router(config)# router bgp 1
   RP/0/RP0/CPU0:router(config-bgp)# address-family ipv4 unicast
   RP/0/RP0/CPU0:router(config-bgp-af)# maximum-paths ibgp 4 selective 
   RP/0/RP0/CPU0:router(config-bgp-af)# maximum-paths ebgp 5 selective 
   RP/0/RP0/CPU0:router(config-bgp-af)# commit
   
   /* To configure selective multipath for eiBGP
   RP/0/RP0/CPU0:router(config)# router bgp 1
   RP/0/RP0/CPU0:router(config-bgp)# address-family ipv4 unicast
   RP/0/RP0/CPU0:router(config-bgp-af)# maximum-paths eibgp 6 selective
   RP/0/RP0/CPU0:router(config-bgp-af)# commit
   

2. Configure neighbors for Router R4.

   Routers R1 (1.1.1.1) and R2 (2.2.2.2) are configured as neighbors with the multipath option.

   Router R3 (3.3.3.3) is configured as a neighbor without the multipath option, and hence the routes from this router are not eligible to be chosen as multipaths.

   
   RP/0/RP0/CPU0:router(config-bgp)# neighbor 1.1.1.1
   RP/0/RP0/CPU0:router(config-bgp-nbr)# address-family ipv4 unicast
   RP/0/RP0/CPU0:router(config-bgp-nbr-af)# multipath     
   RP/0/RP0/CPU0:router(config-bgp-nbr-af)# commit
   
   RP/0/RP0/CPU0:router(config-bgp-nbr)# neighbor 2.2.2.2
   RP/0/RP0/CPU0:router(config-bgp-nbr)# address-family ipv4 unicast
   RP/0/RP0/CPU0:router(config-bgp-nbr-af)# multipath
   RP/0/RP0/CPU0:router(config-bgp-nbr-af)# commit
   
   RP/0/RP0/CPU0:router(config-bgp-nbr)# neighbor 3.3.3.3
   RP/0/RP0/CPU0:router(config-bgp-nbr)# address-family ipv4 unicast
   RP/0/RP0/CPU0:router(config-bgp-nbr-af)# commit
   
   

You have successfully configured the BGP selective multipath feature.

Bi-directional Forwarding Detection Multihop (BFD-MH) support is enabled for BGP. BFD Multihop establishes a BFD session between two addresses that may span multiple network hops. Cisco IOS XR Software BFD Multihop is based on RFC 5883. For more information on BFD Multihop, refer Interface and Hardware Component Configuration Guide for Cisco NCS 6000 Series Routers and Interface and Hardware Component Command Reference for the Cisco NCS 6000 Series Routers.

Multiple BGP instances are supported on the router corresponding to a Autonomous System (AS). Each BGP instance is a separate process running on the same or on a different RP/DRP node. The BGP instances do not share any prefix table between them. No need for a common adj-rib-in (bRIB) as is the case with distributed BGP. The BGP instances do not communicate with each other and do not set up peering with each other. Each individual instance can set up peering with another router independently.

Multi-AS BGP enables configuring each instance of a multi-instance BGP with a different AS number.

Multi-Instance and Multi-AS BGP provides these capabilities:

• 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.

BGP PIC for RIB and FIB adds support for static recursive as PE-CE and faster backup activation by using fast re-route trigger.

The BGP PIC for RIB and FIB feature supports:

• FRR-like trigger for faster PE-CE link down detection, to further reduce the convergence time (Fast PIC-edge activation).

• PIC-edge for static recursive routes.

• BFD single-hop trigger for PIC-Edge without any explicit /32 static route configuration.

• Recursive PIC activation at third level and beyond, on failure trigger at the first (IGP) level.

• BGP path recursion constraints in FIB to ensure that FIB is in sync with BGP with respect to BGP next-hop resolution.

When BGP PIC Edge is configured, configuring the neighbor shutdown command does not trigger CEF to switch to the backup path. Instead, BGP starts to feed CEF again one by one from the top prefix of the routing table to the end thus causing a time delay.

Caution

The time delay causes a traffic outage in the network. As a workaround, you must route the traffic to the backup path manually before configuring the neighbor shutdown command.

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.

The BGP Attribute Filter feature 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 etc. 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.

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).

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

Router# configure
Router(config)# router ospf 100
Router(config-ospf)# distribute link-state instance-id 32

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.

Note	With dynamic route-leaking enabled, BGP bestpath change suppression for eBGP paths might be skipped. BGP convergence might be impacted.

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)#commit

Logging neighbor changes is enabled by default. Use the log neighbor changes disable command to turn off logging. The no log neighbor changes disable command can also be used to turn logging back on if it has been disabled.