Routing Configuration Guide for Cisco NCS 6000 Series Routers, Release 6.1.x
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Border Gateway
Protocol (BGP) is an Exterior Gateway Protocol (EGP) that allows you to create
loop-free interdomain routing between autonomous systems. An
autonomous
system is a set of routers under a single technical administration. Routers
in an autonomous system can use multiple Interior Gateway Protocols (IGPs) to
exchange routing information inside the autonomous system and an EGP to route
packets outside the autonomous system.
This module provides
the conceptual and configuration information for BGP on Cisco IOS XR software.
Note
For more information
about BGP
on the Cisco IOS XR software
and complete descriptions of the BGP commands listed in this
module, see
Related Documents
section of this module. To locate documentation for other commands that might
appear while performing a configuration task, search online in the
Cisco IOS XR
software master command index.
Feature History
for Implementing BGP
Release 5.0.0
This
feature was introduced.
Release 5.2.3
BGP
Nonstop Routing was made a default feature.
Prerequisites for Implementing BGP
You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include
the task IDs required for each command. If you suspect user group assignment is preventing you from using a command, contact
your AAA administrator for assistance.
Information About Implementing BGP
To implement BGP, you need to understand the following concepts:
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
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.
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 Default
Limits
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.
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:
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 onCisco IOS XR
Software module of
Cisco IOS XRRouting
ConfigurationGuide (this
publication).
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 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
address families share the same gateway context, because they are
registered with the IPv4 unicast table in the RIB. As a result,
the global IPv4 unicast table
processed when an IPv4 unicast next-hop notification is received
from the RIB. A mask is maintained in the next hop, indicating
the next
hop belongs to IPv4 unicast. This scoped table walk localizes the processing in the appropriate
address family table.
Reordered Address
Family Processing
The
Cisco IOS XR 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 labeled
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. See the
BGP Commands on
Cisco IOS XR Software
module of
Routing Command Reference for Cisco NCS 6000 Series Routersfor information on the next-hop
show
and
clear commands.
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. See the
BGP Debug Commands
on
Cisco IOS XR Software
module of
.
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.
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:
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 Configuration
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.
Configuration Modes
BGP configurations are grouped into modes. The following sections show how to enter some of the BGP configuration modes. From
a mode, you can enter the ? command to display the commands available in that mode.
Router Configuration Mode
The following example shows how to enter router configuration mode:
Cisco IOS XR BGP uses a neighbor submode to make it
possible to enter configurations without having to prefix every configuration
with the
neighbor keyword and the neighbor address:
Cisco IOS XR software has a submode available for
neighbors in which it is not necessary for every command to have a “neighbor
x.x.x.x” prefix:
In
Cisco IOS XR software, the configuration is as
follows:
An address family
configuration submode inside the neighbor configuration submode is available
for entering address family-specific neighbor configurations. In
Cisco IOS XR software, the configuration is as
follows:
RP/0/RP0/CPU0:router(config-bgp)# neighbor 2002::2RP/0/RP0/CPU0:router(config-bgp-nbr)# remote-as 2023RP/0/RP0/CPU0:router(config-bgp-nbr)# address-family ipv6 unicastRP/0/RP0/CPU0:router(config-bgp-nbr-af)# next-hop-selfRP/0/RP0/CPU0:router(config-bgp-nbr-af)# route-policy one in
Configuration
Templates
The
af-group,
session-group, and neighbor-group
configuration commands provide template support for the neighbor configuration
in
Cisco IOS XR software.
The
af-group command is used to group address
family-specific neighbor commands within an IPv4, IPv6,
address family. Neighbors that have the same
address family configuration are able to use the address family group
(af-group) name for their address family-specific configuration. A neighbor
inherits the configuration from an address family group by way of the
use
command. If a neighbor is configured to use an address family group, the
neighbor (by default) inherits the entire configuration from the address family
group. However, a neighbor does not inherit all of the configuration from the
address family group if items are explicitly configured for the neighbor. The
address family group configuration is entered under the BGP router
configuration mode. The following example shows how to enter address family
group configuration mode
:
The
session-group command allows you to create a session
group from which neighbors can inherit address family-independent
configuration. A neighbor inherits the configuration from a session group by
way of the
use
command. If a neighbor is configured to use a session group, the neighbor (by
default) inherits the entire configuration of the session group. A neighbor
does not inherit all of the configuration from a session group if a
configuration is done directly on that neighbor. The following example shows
how to enter session group configuration mode:
The
neighbor-group command helps you apply the same
configuration to one or more neighbors. Neighbor groups can include session
groups and address family groups and can comprise the complete configuration
for a neighbor. After a neighbor group is configured, a neighbor can inherit
the configuration of the group using the
use
command. If a neighbor is configured to use a neighbor group, the neighbor
inherits the entire BGP configuration of the neighbor group.
The following example
shows how to enter neighbor group configuration mode:
However, a
neighbor does not inherit all of the configuration from the neighbor group if
items are explicitly configured for the neighbor. In addition, some part of the
configuration of the neighbor group could be hidden if a session group or
address family group was also being used.
Configuration grouping
has the following effects in
Cisco IOS XR software:
Commands entered
at the session group level define address family-independent commands (the same
commands as in the neighbor submode).
Commands entered
at the address family group level define address family-dependent commands for
a specified address family (the same commands as in the neighbor-address family
configuration submode).
Commands entered
at the neighbor group level define address family-independent commands and
address family-dependent commands for each address family (the same as all
available
neighbor commands), and define the
use command for the address family group and session
group commands.
Template Inheritance Rules
In Cisco IOS XR software, BGP neighbors or groups inherit configuration from other configuration groups.
For address family-independent configurations:
Neighbors can inherit from session groups and neighbor groups.
Neighbor groups can inherit from session groups and other neighbor groups.
Session groups can inherit from other session groups.
If a neighbor uses a session group and a neighbor group, the configurations in the session group are preferred over the global
address family configurations in the neighbor group.
For address family-dependent configurations:
Address family groups can inherit from other address family groups.
Neighbor groups can inherit from address family groups and other neighbor groups.
Neighbors can inherit from address family groups and neighbor groups.
Configuration group inheritance rules are numbered in order of precedence as follows:
If the item is configured directly on the neighbor, that value is used. In the example that follows, the advertisement interval
is configured both on the neighbor group and neighbor configuration and the advertisement interval being used is from the
neighbor configuration:
The following output from the show bgp neighbors command shows that the advertisement interval used is 20 seconds:
RP/0/RP0/CPU0:router# show bgp neighbors 10.1.1.1
BGP neighbor is 10.1.1.1, remote AS 1, local AS 140, external link
Remote router ID 0.0.0.0
BGP state = Idle
Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Minimum time between advertisement runs is 20 seconds
For Address Family: IPv4 Unicast
BGP neighbor version 0
Update group: 0.1
eBGP neighbor with no inbound or outbound policy; defaults to 'drop'
Route refresh request: received 0, sent 0
0 accepted prefixes
Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
Threshold for warning message 75%
Connections established 0; dropped 0
Last reset 00:00:14, due to BGP neighbor initialized
External BGP neighbor not directly connected.
Otherwise, if an item is configured to be inherited from a session-group or neighbor-group and on the neighbor directly, then
the configuration on the neighbor is used. If a neighbor is configured to be inherited from session-group or af-group, but
no directly configured value, then the value in the session-group or af-group is used. In the example that follows, the advertisement
interval is configured on a neighbor group and a session group and the advertisement interval value being used is from the
session group:
The following output from the show bgp neighbors command shows that the advertisement interval used is 15 seconds:
RP/0/RP0/CPU0:router# show bgp neighbors 192.168.0.1
BGP neighbor is 192.168.0.1, remote AS 1, local AS 140, external link
Remote router ID 0.0.0.0
BGP state = Idle
Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Minimum time between advertisement runs is 15 seconds
For Address Family: IPv4 Unicast
BGP neighbor version 0
Update group: 0.1
eBGP neighbor with no inbound or outbound policy; defaults to 'drop'
Route refresh request: received 0, sent 0
0 accepted prefixes
Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
Threshold for warning message 75%
Connections established 0; dropped 0
Last reset 00:03:23, due to BGP neighbor initialized
External BGP neighbor not directly connected.
Otherwise, if the neighbor uses a neighbor group and does not use a session group or address family group, the configuration
value can be obtained from the neighbor group either directly or through inheritance. In the example that follows, the advertisement
interval from the neighbor group is used because it is not configured directly on the neighbor and no session group is used:
The following output from the show bgp neighbors command shows that the advertisement interval used is 15 seconds:
RP/0/RP0/CPU0:router# show bgp neighbors 192.168.1.1
BGP neighbor is 192.168.2.2, remote AS 1, local AS 140, external link
Remote router ID 0.0.0.0
BGP state = Idle
Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Minimum time between advertisement runs is 15 seconds
For Address Family: IPv4 Unicast
BGP neighbor version 0
Update group: 0.1
eBGP neighbor with no outbound policy; defaults to 'drop'
Route refresh request: received 0, sent 0
Inbound path policy configured
Policy for incoming advertisements is POLICY_1
0 accepted prefixes
Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
Threshold for warning message 75%
Connections established 0; dropped 0
Last reset 00:01:14, due to BGP neighbor initialized
External BGP neighbor not directly connected.
To illustrate the same rule, the following example shows how to set the advertisement interval to 15 (from the session group)
and 25 (from the neighbor group). The advertisement interval set in the session group overrides the one set in the neighbor
group. The inbound policy is set to POLICY_1 from the neighbor group.
The following output from the show bgp neighbors command shows that the advertisement interval used is 15 seconds:
RP/0/RP0/CPU0:router# show bgp neighbors 192.168.2.2
BGP neighbor is 192.168.2.2, remote AS 1, local AS 140, external link
Remote router ID 0.0.0.0
BGP state = Idle
Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Minimum time between advertisement runs is 15 seconds
For Address Family: IPv4 Unicast
BGP neighbor version 0
Update group: 0.1
eBGP neighbor with no inbound or outbound policy; defaults to 'drop'
Route refresh request: received 0, sent 0
0 accepted prefixes
Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
Threshold for warning message 75%
Connections established 0; dropped 0
Last reset 00:02:03, due to BGP neighbor initialized
External BGP neighbor not directly connected.
Otherwise, the default value is used. In the example that follows, neighbor 10.0.101.5 has the minimum time between advertisement
runs set to 30 seconds (default) because the neighbor is not configured to use the neighbor configuration or the neighbor
group configuration:
The following output from the show bgp neighbors command shows that the advertisement interval used is 30 seconds:
RP/0/RP0/CPU0:router# show bgp neighbors 10.0.101.5
BGP neighbor is 10.0.101.5, remote AS 1, local AS 140, external link
Remote router ID 0.0.0.0
BGP state = Idle
Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Minimum time between advertisement runs is 30 seconds
For Address Family: IPv4 Unicast
BGP neighbor version 0
Update group: 0.2
eBGP neighbor with no inbound or outbound policy; defaults to 'drop'
Route refresh request: received 0, sent 0
0 accepted prefixes
Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
Threshold for warning message 75%
Connections established 0; dropped 0
Last reset 00:00:25, due to BGP neighbor initialized
External BGP neighbor not directly connected.
The inheritance rules used when groups are inheriting configuration from other groups are the same as the rules given for
neighbors inheriting from groups.
Viewing Inherited Configurations
You can use the following show commands to view BGP inherited configurations:
show bgp
neighbors
Use the
show bgp
neighbors
command to display information about the BGP configuration for
neighbors.
Use the
configuration keyword to display the effective
configuration for the neighbor, including any settings that have been inherited
from session groups, neighbor groups, or address family groups used by this
neighbor.
Use the
inheritance keyword to display the session groups,
neighbor groups, and address family groups from which this neighbor is capable
of inheriting configuration.
The
show bgp
neighbors command examples that follow are based on this sample
configuration:
Use the show bgp af-group command to display address family groups:
Use the configuration keyword to display the effective configuration for the address family group, including any settings that have been inherited
from address family groups used by this address family group.
Use the inheritance keyword to display the address family groups from which this address family group is capable of inheriting configuration.
Use the users keyword to display the neighbors, neighbor groups, and address family groups that inherit configuration from this address
family group.
The show bgp af-group sample commands that follow are based on this sample configuration:
The following example displays sample output from the show bgp af-group command using the configuration keyword. This example shows from where each configuration item was inherited. The default-originate command was configured directly on this address family group (indicated by [ ]). The remove-private-as command was inherited from address family group GROUP_2, which in turn inherited from address family group GROUP_3:
The following example displays sample output from the show bgp af-group command using the users keyword:
RP/0/RP0/CPU0:router# show bgp af-group GROUP_2 users
IPv4 Unicast: a:GROUP_1
The following example displays sample output from the show bgp af-group command using the inheritance keyword. This shows that the specified address family group GROUP_1 directly uses the GROUP_2 address family group, which
in turn uses the GROUP_3 address family group:
RP/0/RP0/CPU0:router# show bgp af-group GROUP_1 inheritance
IPv4 Unicast: a:GROUP_2 a:GROUP_3
show bgp
session-group
Use the
show bgp
session-group command to display session groups:
Use the
configuration keyword to display the effective
configuration for the session group, including any settings that have been
inherited from session groups used by this session group.
Use the
inheritance keyword to display the session groups from
which this session group is capable of inheriting configuration.
Use the
users keyword to display the session groups, neighbor
groups, and neighbors that inherit configuration from this session group.
The output from the
show bgp
session-group command is based on the following session group
configuration:
The following is
sample output from the
show bgp
session-group command with the
inheritance keyword showing that the GROUP_1 session group inherits session
parameters from the GROUP_3 and GROUP_2 session groups:
RP/0/RP0/CPU0:router# show bgp session-group GROUP_1 inheritance
Session: s:GROUP_2 s:GROUP_3
The following is
sample output from the
show bgp
session-group command with the
users keyword showing that both the GROUP_1 and
GROUP_2 session groups inherit session parameters from the GROUP_3 session
group:
RP/0/RP0/CPU0:router# show bgp session-group GROUP_3 users
Session: s:GROUP_1 s:GROUP_2
show bgp neighbor-group
Use the show bgp neighbor-group command to display neighbor groups:
Use the configuration keyword to display the effective configuration for the neighbor group, including any settings that have been inherited from
neighbor groups used by this neighbor group.
Use the inheritance keyword to display the address family groups, session groups, and neighbor groups from which this neighbor group is capable
of inheriting configuration.
Use the users keyword to display the neighbors and neighbor groups that inherit configuration from this neighbor group.
The examples are based on the following group configuration:
The following is sample output from the show bgp neighbor-group command with the configuration keyword. The configuration setting source is shown to the right of each command. In the output shown previously, the remote
autonomous system is configured directly on neighbor group GROUP_1, and the send community setting is inherited from neighbor
group GROUP_2, which in turn inherits the setting from address family group GROUP_3:
The following is sample output from the show bgp neighbor-group command with the inheritance keyword. This output shows that the specified neighbor group GROUP_1 inherits session (address family-independent) configuration
parameters from neighbor group GROUP_2. Neighbor group GROUP_2 inherits its session parameters from session group GROUP_3.
It also shows that the GROUP_1 neighbor group inherits IPv4 unicast configuration parameters from the GROUP_2 neighbor group,
which in turn inherits them from the GROUP_2 address family group, which itself inherits them from the GROUP_3 address family
group:
The following is sample output from the show bgp neighbor-group command with the users keyword. This output shows that the GROUP_1 neighbor group inherits session (address family-independent) configuration parameters
from the GROUP_2 neighbor group. The GROUP_1 neighbor group also inherits IPv4 unicast configuration parameters from the GROUP_2
neighbor group:
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.
Neighbor Address Family Combinations
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.
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.
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:
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:
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
Table Policy
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.
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.
To use this feature, you must understand the following concepts:
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
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
How BGP Cost Community Influences the Best Path Selection Process
The cost community attribute influences the BGP best-path selection process at the point of insertion (POI). By default, the
POI follows the Interior Gateway Protocol (IGP) metric comparison. When BGP receives multiple paths to the same destination,
it uses the best-path selection process to determine which path is the best path. BGP automatically makes the decision and
installs the best path in the routing table. The POI allows you to assign a preference to a specific path when multiple equal
cost paths are available. If the POI is not valid for local best-path selection, the cost community attribute is silently
ignored.
Cost communities are sorted first by POI then by community ID. Multiple paths can be configured with the cost community attribute
for the same POI. The path with the lowest cost community ID is considered first. In other words, all cost community paths
for a specific POI are considered, starting with the one with the lowest cost community. Paths that do not contain the cost
community cost (for the POI and community ID being evaluated) are assigned the default community cost value (2147483647).
If the cost community values are equal, then cost community comparison proceeds to the next lowest community ID for this POI.
To select the path with the lower cost community, simultaneously walk through the cost communities of both paths. This is
done by maintaining two pointers to the cost community chain, one for each path, and advancing both pointers to the next applicable
cost community at each step of the walk for the given POI, in order of community ID, and stop when a best path is chosen or
the comparison is a tie. At each step of the walk, the following checks are done:
If neither pointer refers to a cost community,
Declare a tie;
Elseif a cost community is found for one path but not for the other,
Choose the path with cost community as best path;
Elseif the Community ID from one path is less than the other,
Choose the path with the lesser Community ID as best path;
Elseif the Cost from one path is less than the other,
Choose the path with the lesser Cost as best path;
Else Continue.
Note
Paths that are not configured with the cost community attribute are considered by the best-path selection process to have
the default cost value (half of the maximum value [4294967295] or 2147483647).
Applying the cost community attribute at the POI allows you to assign a value to a path originated or learned by a peer in
any part of the local autonomous system or confederation. The cost community can be used as a “tie breaker” during the best-path
selection process. Multiple instances of the cost community can be configured for separate equal cost paths within the same
autonomous system or confederation. For example, a lower cost community value can be applied to a specific exit path in a
network with multiple equal cost exit points, and the specific exit path is preferred by the BGP best-path selection process.
See the scenario described inInfluencing Route Preference in a Multiexit IGP Network.
Note
The cost community comparison in BGP is enabled by default. Use the bgp bestpath cost-community ignore command to disable the comparison.
Cost Community Support for Aggregate Routes and Multipaths
The BGP cost community feature supports aggregate routes and multipaths. The cost community attribute can be applied to either
type of route. The cost community attribute is passed to the aggregate or multipath route from component routes that carry
the cost community attribute. Only unique IDs are passed, and only the highest cost of any individual component route is applied
to the aggregate for each ID. If multiple component routes contain the same ID, the highest configured cost is applied to
the route. For example, the following two component routes are configured with the cost community attribute using an inbound
route policy:
10.0.0.1
POI=IGP
cost community ID=1
cost number=100
192.168.0.1
POI=IGP
cost community ID=1
cost number=200
If these component routes are aggregated or configured as a multipath, the cost value 200 is advertised, because it has the
highest cost.
If one or more component routes do not carry the cost community attribute or the component routes are configured with different
IDs, then the default value (2147483647) is advertised for the aggregate or multipath route. For example, the following three
component routes are configured with the cost community attribute using an inbound route policy. However, the component routes
are configured with two different IDs.
10.0.0.1
POI=IGP
cost community ID=1
cost number=100
172.16.0.1
POI=IGP
cost community ID=2
cost number=100
192.168.0.1
POI=IGP
cost community ID=1
cost number=200
The single advertised path includes the aggregate cost communities as follows:
Influencing Route Preference in a Multiexit IGP Network
This figure
shows an IGP network with two autonomous system boundary routers (ASBRs) on the edge. Each ASBR has an equal cost path to
network 10.8/16.
Figure 1. Multiexit Point IGP Network
Both paths are considered to be equal by BGP. If multipath loadsharing is configured, both paths to the routing table are
installed and are used to balance the load of traffic. If multipath load balancing is not configured, the BGP selects the
path that was learned first as the best path and installs this path to the routing table. This behavior may not be desirable
under some conditions. For example, the path is learned from ISP1 PE2 first, but the link between ISP1 PE2 and ASBR1 is a
low-speed link.
The configuration of the cost community attribute can be used to influence the BGP best-path selection process by applying
a lower-cost community value to the path learned by ASBR2. For example, the following configuration is applied to ASBR2:
RP/0/RP0/CPU0:router(config)# route-policy ISP2_PE1RP/0/RP0/CPU0:router(config-rpl)# set extcommunity cost (1:1)
The preceding route policy applies a cost community number of 1 to the 10.8.0.0 route. By default, the path learned from ASBR1
is assigned a cost community number of 2147483647. Because the path learned from ASBR2 has a lower-cost community number,
the path is preferred.
Adding Routes to the Routing Information Base
If a nonsourced path becomes the best path after the best-path calculation, BGP adds the route to the Routing Information
Base (RIB) and passes the cost communities along with the other IGP extended communities.
When a route with paths is added to the RIB by a protocol, RIB checks the current best paths for the route and the added paths
for cost extended communities. If cost-extended communities are found, the RIB compares the set of cost communities. If the
comparison does not result in a tie, the appropriate best path is chosen. If the comparison results in a tie, the RIB proceeds
with the remaining steps of the best-path algorithm. If a cost community is not present in either the current best paths or
added paths, then the RIB continues with the remaining steps of the best-path algorithm. See BGP Best Path Algorithm for information on the BGP best-path algorithm.
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.
Administrative Distance
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.
Table 1. BGP Default Administrative Distances
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.
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 Figure 1.
Figure 2. Back Door Example
In Figure 1, 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:
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
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.
Minimizing Flapping
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.
BGP Routing Domain Confederation
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 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.
Figure 1
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.
Figure 3. Three Fully Meshed iBGP Speakers
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 Figure 2 , 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.
Figure 4. Simple BGP Model with a Route Reflector
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.
Figure 5. More Complex BGP Route Reflector Model
Figure 3
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 Blackhole Filtering with RPL Next-hop Discard Configuration
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
Configuring Destination-based RTBH Filtering
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.
Figure 6. Topology to Implement RTBH Filtering
Configurations applied on the Trigger Router
Configure a static route redistribution policy that sets a community on static routes marked with a special tag, and apply
it in BGP:
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
Configure a static route with the special tag for the source prefix that has to be block-holed:
router static
address-family ipv4 unicast
10.7.7.7/32 Null0 tag 777
Configurations applied on the Border Router
Configure a route policy that matches the community set on the trigger router and configure set next-hop discard:
route-policy RTBH
if community matches-any (1234:4321) then
set next-hop discard
else
pass
endif
end-policy
Apply the route policy on the iBGP peers:
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
Verification
On the border router, the prefix 10.7.7.7/32 is flagged as Nexthop-discard:
RP/0/RSP0/CPU0:router#show bgp
BGP router identifier 10.210.0.5, local AS number 65001
BGP generic scan interval 60 secs
BGP table state: Active
Table ID: 0xe0000000 RD version: 12
BGP main routing table version 12
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
N>i10.7.7.7/32 192.168.102.2 0 100 0 ?
RP/0/RSP0/CPU0:router#show bgp 10.7.7.7/32
BGP routing table entry for 10.7.7.7/32
Versions:
Process bRIB/RIB SendTblVer
Speaker 12 12
Last Modified: Jul 4 14:37:29.048 for 00:20:52
Paths: (1 available, best #1, not advertised to EBGP peer)
Not advertised to any peer
Path #1: Received by speaker 0
Not advertised to any peer
Local
192.168.102.2 (discarded) from 192.168.102.2 (10.210.0.2)
Origin incomplete, metric 0, localpref 100, valid, internal best, group-best
Received Path ID 0, Local Path ID 1, version 12
Community: 1234:4321 no-export
RP/0/RSP0/CPU0:router#show route 10.7.7.7/32
Routing entry for 10.7.7.7/32
Known via "bgp 65001", distance 200, metric 0, type internal
Installed Jul 4 14:37:29.394 for 01:47:02
Routing Descriptor Blocks
directly connected, via Null0
Route metric is 0
No advertising protos.
Default Address
Family for show Commands
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
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 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 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.
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.
BGP Best-External
Path
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:
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
The
advertise
best-external
command enables the advertisement of the best–external path in
global address family configuration mode.
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.
The retain local-label command enables the retention of the local label until the network is converged.
BGP Over GRE
Interfaces
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.
Command Line
Interface (CLI) Consistency for BGP Commands
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.
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.
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.
BGP Selective
Multipath
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.
Figure 7. BGP Selective Multipath
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.
Configure
Router R4 to accept selective multiple paths in your topology.
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.
You have
successfully configured the BGP selective multipath feature.
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.
BFD Multihop Support for BGP
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.
BGP Multi-Instance
and Multi-AS
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.
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.
BGP Prefix Independent Convergence for RIB and FIB
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 black hole in the network. As a workaround, you must route the traffic to the backup path manually
before configuring the neighbor shutdown command.
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 Attribute Filtering
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 Attribute Filter
Actions
The
Attribute-filtering is configured by specifying a single or a range of
attribute codes and an associated action. The allowed actions are:
"
Treat-as-withdraw"— The associated IPv4-unicast or MP_REACH NLRIs, if present,
are withdrawn from the neighbor's Adj-RIB-In.
"Discard
Attribute"—The matching attributes alone are discarded and the rest of the
Update message is processed normally.
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.
Use the
attribute-filter
group command to enter Attribute-filter group command mode. Use
the
attribute
command in attribute-filter group command mode to either discard an attribute
or treat the update message as a "Withdraw" action.
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.
Note
Directly connected routes cannot be leaked using BGP VRF Dynamic Route Leaking from default VRF to non-default VRF
The dynamic route
leaking is enabled by:
Importing from
default-VRF to non-default-VRF, using the
import from
default-vrf
route-policyroute-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-vrfroute-policyroute-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.
How to Implement BGP
Enabling 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
At least one
neighbor and at least one address family must be configured to enable BGP
routing. At least one neighbor with both a remote AS and an address family must
be configured globally using the
address
family and
remote
as commands.
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
While establishing eBGP neighborship between two peers, BGP checks if the two peers are directly connected. If the peers are
not directly connected, BGP does not try to establish a relationship by default. If two BGP peers are not directly connected
and peering is required between the loop backs of the routers, you can use the ignore-connected-check command. This command overrides the default check that BGP performs which is to verify if source IP in BGP control packets
is in same network as that of destination. In this scenario, a TTL value of 1 is sufficient if ignore-connected-check is used.
Configuring egp-multihopttl is needed when the peers are not directly connected and there are more routers in between. If the egp-multihopttl command is not configured, eBGP sets the TTL of packets carrying BGP messages to 1 by default. When eBGP needs to be setup
between routers which are more than one hop away, you need to configure a TTL value which is at least equal to the number
of hops between them. For example, if there are 2 hops (R2, R3) between two BGP peering routers R1 and R4, you need to set
a TTL value of 3.
SUMMARY STEPS
configure
route-policyroute-policy-name
end-policy
commit
configure
router bgp
as-number
bgp
router-idip-address
address-family {ipv4 |
ipv6}
unicast
exit
neighborip-address
remote-asas-number
address-family {ipv4 |
ipv6}
unicast
route-policyroute-policy-name {in |
out}
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
route-policyroute-policy-name
Example:
RP/0/RP0/CPU0:router(config)# route-policy drop-as-1234
RP/0/RP0/CPU0:router(config-rpl)# if as-path passes-through '1234' then
RP/0/RP0/CPU0:router(config-rpl)# apply check-communities
RP/0/RP0/CPU0:router(config-rpl)# else
RP/0/RP0/CPU0:router(config-rpl)# pass
RP/0/RP0/CPU0:router(config-rpl)# endif
(Optional)
Creates a route policy and enters route policy configuration mode, where you
can define the route policy.
Step 3
end-policy
Example:
RP/0/RP0/CPU0:router(config-rpl)# end-policy
(Optional) Ends
the definition of a route policy and exits route policy configuration mode.
Step 4
commit
Step 5
configure
Step 6
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120
Specifies the
BGP AS number and enters the BGP configuration mode, allowing you to configure
the BGP routing process.
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 13
route-policyroute-policy-name {in |
out}
Example:
RP/0/RP0/CPU0:router(config-bgp-nbr-af)# route-policy drop-as-1234 in
(Optional)
Applies the specified policy to inbound IPv4 unicast routes.
Step 14
commit
Configuring 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.
Configures a fixed router ID for the BGP-speaking router (BGP instance).
Note
You must manually configure unique router ID for each BGP instance.
Step 4
commit
Configuring a
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.
SUMMARY STEPS
configure
router bgp
as-number
bgp
confederation identifieras-number
bgp
confederation peersas-number
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router# router bgp 120
Specifies the
autonomous system number and enters the BGP configuration mode, allowing you to
configure the BGP routing process.
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
commit
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.
eBGP sessions flap when the node reaches 3500 eBGP sessions with BGP timer values set as 10 and 30.
To support more than 3500 eBGP sessions, increase the packet
rate by using the lpts pifib hardware police locationlocation-id command. Following is a sample configuration to increase the eBGP sessions:
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.
Adjusting BGP
Timers
Perform this task to
set the timers for BGP neighbors.
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.
SUMMARY STEPS
configure
router bgp
as-number
timers
bgpkeepalive
hold-time
neighborip-address
timerskeepalive
hold-time
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 123
Specifies the
autonomous system number and enters the BGP configuration mode, allowing you to
configure the BGP routing process.
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).
Step 4
commit
Configuring the 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).
SUMMARY STEPS
configure
router bgp
as-number
default-metricvalue
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120
Specifies the
autonomous system number and enters the BGP configuration mode, allowing you to
configure the BGP routing process.
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).
Step 4
commit
Configuring BGP
Weights
Perform this task to
assign a weight to routes received from a neighbor. 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.
Before you begin
Note
The
clear
bgp command must be used for the newly configured weight to take
effect.
SUMMARY STEPS
configure
router bgp
as-number
neighborip-address
remote-asas-number
address-family {ipv4 |
ipv6}
unicast
weightweight-value
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120
Specifies the
autonomous system number and enters the BGP configuration mode, allowing you to
configure the BGP routing process.
Assigns a weight
to all routes learned through the neighbor.
Step 7
commit
Tuning the BGP
Best-Path Calculation
Perform this task to
change the default BGP best-path calculation behavior.
SUMMARY STEPS
configure
router bgp
as-number
bgp bestpath
med missing-as-worst
bgp bestpath
med always
bgp bestpath
med confed
bgp bestpath
as-path ignore
bgp bestpath
compare-routerid
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 126
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:
RP/0/RP0/CPU0:router(config-bgp)# bgp bestpath med missing-as-worst
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.
Step 4
bgp bestpath
med always
Example:
RP/0/RP0/CPU0:router(config-bgp)# bgp bestpath med always
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.
Step 5
bgp bestpath
med confed
Example:
RP/0/RP0/CPU0:router(config-bgp)# bgp bestpath med confed
Enables BGP
software to compare MED values for paths learned from confederation peers.
Configure the
BGP speaker in the autonomous system to compare the router IDs of similar
paths.
Step 8
commit
Indicating BGP
Back-door Routes
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.
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.
The
as-set keyword generates autonomous system set path
information and community information from contributing paths.
The
as-confed-set
keyword generates autonomous system confederation set
path information from contributing paths.
The
summary-only keyword filters all more specific routes from updates.
The
route-policyroute-policy-name keyword and argument specify the route policy used to
set the attributes of the aggregate route.
Step 5
commit
Redistributing 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.
SUMMARY STEPS
configure
router bgp
as-number
bgp
redistribute-internal
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120
Specifies the
autonomous system number and enters the BGP configuration mode, allowing you to
configure the BGP routing process.
Allows the
redistribution of iBGP routes into an IGP, such as IS-IS or OSPF.
Step 4
commit
Redistributing
Prefixes into Multiprotocol BGP
Perform this task to
redistribute prefixes from another protocol into multiprotocol BGP.
Redistribution is
the process of injecting prefixes from one routing protocol into another
routing protocol. This task shows how to inject prefixes from another routing
protocol into multiprotocol BGP. Specifically, prefixes that are redistributed
into multiprotocol BGP using the
redistribute
command are injected into the unicast database.
Note
BGP doesn’t support redistribution of ISIS routes in VRF.
Configures BGP
dampening for the specified address family.
Step 5
commit
Applying Policy When
Updating the Routing Table
Perform this task to
apply a routing policy to routes being installed into the routing table.
Before you begin
See the
Implementing
Routing Policy on
module of
Routing Configuration Guide for Cisco NCS 6000 Series Routers (this publication) for a list of the
supported attributes and operations that are valid for table policy filtering.
SUMMARY STEPS
configure
router bgp
as-number
address-family {ipv4 |
ipv6}
unicast
table-policypolicy-name
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120.6
Specifies the
autonomous system number and enters the BGP configuration mode, allowing you to
configure the BGP routing process.
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
commit
Configuring a 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 associated with the neighbor.
After a neighbor
group is configured, each neighbor can inherit the configuration through the
use
command. 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
use
command.
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-family
command 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.
SUMMARY STEPS
configure
router bgp
as-number
address-family {ipv4 |
ipv6}
unicast
exit
neighbor-groupname
remote-asas-number
address-family {ipv4 |
ipv6}
unicast
route-policyroute-policy-name {in |
out}
exit
exit
neighborip-address
use
neighbor-groupgroup-name
remote-asas-number
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120
Specifies the
autonomous system number and enters the BGP configuration mode, allowing you to
configure the BGP routing process.
Creates a
neighbor and assigns a remote autonomous system number to it.
Step 14
commit
Configuring a 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.
SUMMARY STEPS
configure
router bgp
as-number
bgp
cluster-idcluster-id
neighborip-address
remote-asas-number
address-family {ipv4 |
ipv6}
unicast
route-reflector-client
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120
Specifies the
autonomous system number and enters the BGP configuration mode, allowing you to
configure the BGP routing process.
Configures the
router as a BGP route reflector and configures the neighbor as its client.
Step 8
commit
Configuring BGP
Route Filtering by Route Policy
Perform this task to
configure BGP routing filtering by route policy.
Before you begin
See the
Implementing
Routing Policy on
module of
Cisco
Routing
Configuration Guide (this publication) for a list of the supported
attributes and operations that are valid for inbound and outbound neighbor
policy filtering.
SUMMARY STEPS
configure
route-policyname
end-policy
router bgp
as-number
neighborip-address
address-family {ipv4 |
ipv6}
unicast
route-policyroute-policy-name {in |
out}
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
route-policyname
Example:
RP/0/RP0/CPU0:router(config)# route-policy drop-as-1234
RP/0/RP0/CPU0:router(config-rpl)# if as-path passes-through '1234' then
RP/0/RP0/CPU0:router(config-rpl)# apply check-communities
RP/0/RP0/CPU0:router(config-rpl)# else
RP/0/RP0/CPU0:router(config-rpl)# pass
RP/0/RP0/CPU0:router(config-rpl)# endif
(Optional)
Creates a route policy and enters route policy configuration mode, where you
can define the route policy.
Step 3
end-policy
Example:
RP/0/RP0/CPU0:router(config-rpl)# end-policy
(Optional) Ends
the definition of a route policy and exits route policy configuration mode.
Step 4
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120
Specifies the
autonomous system number and enters the BGP configuration mode, allowing you to
configure the BGP routing process.
Specifies the autonomous system number and enters the BGP
configuration mode, allowing you to configure the BGP routing process.
Step 3
attribute-filter groupattribute-filter group name
Example:
RP/0/RP0/CPU0:router(config-bgp)# attribute-filter group ag_discard_med
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.
Specifies a single or a range of attribute codes and an associated
action. The allowed actions are:
Treat-as-withdraw— Considers the update message for
withdrawal. The associated IPv4-unicast or MP_REACH NLRIs, if present, are
withdrawn from the neighbor's Adj-RIB-In.
Discard Attribute— Discards this attribute. The matching
attributes alone are discarded and the rest of the Update message is processed
normally.
Configuring 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.
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.
SUMMARY STEPS
configure
router bgp
as-number
neighborip-address
remote-asas-number
address-family {ipv4 |
ipv6}
unicast
next-hop-self
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120
Specifies the
autonomous system number and enters the BGP configuration mode, allowing you to
configure the BGP routing process.
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.
Step 7
commit
Configuring 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
IPv4, and IPv6 address families.
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.
IPv6 address
family mode supports these sub modes:
labeled-unicast
unicast
IPv4 address
family mode supports these sub modes:
labeled-unicast
unicast
Refer the
address-family
(BGP) command in
BGP
Commands module of
Routing Command Reference for Cisco NCS 6000 Series Routers for more information on the
Address Family Submode support.
Specifies that
the router send community attributes or extended community attributes (which
are disabled by default for eBGP neighbors) to a specified eBGP neighbor.
Step 7
commit
Configuring the BGP
Cost Community
Perform this task to
configure the 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.
SUMMARY STEPS
configure
route-policyname
set
extcommunity cost
{cost-extcommunity-set-name |
cost-inline-extcommunity-set} [additive]
Displays the
cost community in the following format:
Cost:POI:cost-community-ID:cost-number
Configuring Software
to Store Updates from a 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. See the
Resetting Neighbors Using BGP Inbound Soft Reset.
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.
SUMMARY STEPS
configure
router bgp
as-number
neighborip-address
address-family {ipv4 |
ipv6}
unicast
soft-reconfiguration inbound
[always]
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120
Specifies the
autonomous system number and enters the BGP configuration mode, allowing you to
configure the BGP routing process.
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.
Step 6
commit
Configuring
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.
SUMMARY STEPS
configure
router bgp
as-number
neighborip-address
remote-asas-number
keychainname
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120
Specifies the
autonomous system number and enters the BGP configuration mode, allowing you to
configure the BGP routing process.
Places the
router in neighbor configuration mode for BGP routing and configures the
neighbor IP address as a BGP peer.
Step 4
shutdown
Example:
RP/0/RP0/CPU0:router(config-bgp-nbr)# shutdown
Disables all
active sessions for the specified neighbor.
Step 5
commit
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.
RP/0/RP0/CPU0:router# clear bgp ipv4 unicast 10.0.0.1 soft in
Soft resets a
BGP neighbor.
The
* keyword resets all BGP neighbors.
The
ip-address argument specifies the address of the neighbor to be
reset.
The
as-number argument specifies that all neighbors that match the
autonomous system number be reset.
The
external keyword specifies that all external neighbors are
reset.
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.
SUMMARY STEPS
show bgp
neighbors
out
DETAILED STEPS
Command or Action
Purpose
Step 1
show bgp
neighbors
Example:
RP/0/RP0/CPU0:router# show bgp neighbors
Verifies that
received route refresh capability from the neighbor is enabled.
Step 2
out
Example:
RP/0/RP0/CPU0:router# clear bgp ipv4 unicast 10.0.0.2 soft out
Soft resets a
BGP neighbor.
The
* keyword resets all BGP neighbors.
The
ip-address argument specifies the address of the neighbor to be
reset.
The
as-number argument specifies that all neighbors that match the
autonomous system number be reset.
The
external keyword specifies that all external neighbors are
reset.
Resetting 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.
Perform this task to
remove all contents of a particular cache, table, or database. The
clear
bgp command resets the sessions of the specified group of neighbors
(hard reset); it 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. Clearing a cache, table, or database can become
necessary when the contents of the particular structure have become, or are
suspected to be, invalid.
Perform this task to display specific statistics, such as the contents of BGP routing tables, caches, and databases. Information
provided can be used to determine resource usage and solve network problems. You can also display information about node reachability
and discover the routing path that the packets of your device are taking through the network.
SUMMARY STEPS
show bgp cidr-only
show bgp communitycommunity-list [exact-match]
show bgpregexpregular-expression
show bgp
show bgp neighborsip-address [advertised-routes | dampened-routes | flap-statistics | performance-statistics | received prefix-filter | routes]
RP/0/RP0/CPU0:router# show bgp community 1081:5 exact-match
Displays routes that match the specified BGP community.
Step 3
show bgpregexpregular-expression
Example:
RP/0/RP0/CPU0:router# show bgp regexp "^3 "
Displays routes that match the specified autonomous system path regular expression.
Step 4
show bgp
Example:
RP/0/RP0/CPU0:router# show bgp
Displays entries in the BGP routing table.
Step 5
show bgp neighborsip-address [advertised-routes | dampened-routes | flap-statistics | performance-statistics | received prefix-filter | routes]
Example:
RP/0/RP0/CPU0:router# show bgp neighbors 10.0.101.1
Displays information about the BGP connection to the specified neighbor.
The advertised-routes keyword displays all routes the router advertised to the neighbor.
The dampened-routes keyword displays the dampened routes that are learned from the neighbor.
The flap-statistics keyword displays flap statistics of the routes learned from the neighbor.
The performance-statistics keyword displays performance statistics relating to work done by the BGP process for this neighbor.
The receivedprefix-filter keyword and argument display the received prefix list filter.
The routes keyword displays routes learned from the neighbor.
Step 6
show bgp paths
Example:
RP/0/RP0/CPU0:router# show bgp paths
Displays all BGP paths in the database.
Step 7
show bgp neighbor-groupgroup-nameconfiguration
Example:
RP/0/RP0/CPU0:router# show bgp neighbor-group group_1 configuration
Displays the effective configuration for a specified neighbor group, including any configuration inherited by this neighbor
group.
Step 8
show bgp summary
Example:
RP/0/RP0/CPU0:router# show bgp summary
Displays the status of all BGP connections.
Displaying BGP
Process Information
Perform this task to
display specific BGP process information.
SUMMARY STEPS
show bgp
process
show bgp
ipv4 unicast summary
show bgp
process detail
show bgp
summary
show
placement program bgp
show
placement program brib
DETAILED STEPS
Command or Action
Purpose
Step 1
show bgp
process
Example:
RP/0/RP0/CPU0:router# show bgp process
Displays status
and summary information for the BGP process. The output shows various global
and address family-specific BGP configurations. A summary of the number of
neighbors, update messages, and notification messages sent and received by the
process is also displayed.
Step 2
show bgp
ipv4 unicast summary
Example:
RP/0/RP0/CPU0:router# show bgp ipv4 unicast summary
Displays a
summary of the neighbors for the IPv4 unicast address family.
Step 3
show bgp
process detail
Example:
RP/0/RP0/CPU0:router# show bgp processes detail
Displays
detailed process information including the memory used by each of various
internal structure types.
Step 4
show bgp
summary
Example:
RP/0/RP0/CPU0:router# show bgp summary
Displays the
status of all BGP connections.
Step 5
show
placement program bgp
Example:
RP/0/RP0/CPU0:router# show placement program bgp
Displays BGP
program information.
If a program
is shown as having ‘rejected locations’ (for example, locations where program
cannot be placed), the locations in question can be viewed using the
show
placement program bgp command.
If a program
has been placed but not started, the amount of elapsed time since the program
was placed is displayed in the Waiting to start column.
Step 6
show
placement program brib
Example:
RP/0/RP0/CPU0:router# show placement program brib
Displays bRIB
program information.
If a program
is shown as having ‘rejected locations’ (for example, locations where program
cannot be placed), the locations in question can be viewed using the
show
placement program bgp command.
If a
program has been placed but not started, the amount of elapsed time since the
program was placed is displayed in the Waiting to start column.
Monitoring BGP
Update Groups
This task displays
information related to the processing of BGP update groups.
SUMMARY STEPS
show
bgpupdate-group
[neighborip-address |
process-id.index
[summary|performance-statistics]]
DETAILED STEPS
Command or Action
Purpose
show
bgpupdate-group
[neighborip-address |
process-id.index
[summary|performance-statistics]]
Example:
RP/0/RP0/CPU0:router# show bgp update-group 0.0
Displays
information about BGP update groups.
The
ip-address argument displays the update groups to which that
neighbor belongs.
The
process-id.index argument selects a particular update group to display
and is specified as follows: process ID (dot) index. Process ID range is from 0
to 254. Index range is from 0 to 4294967295.
The
summary keyword displays summary information for neighbors
in a particular update group.
If no
argument is specified, this command displays information for all update groups
(for the specified address family).
The
performance-statistics keyword displays performance
statistics for an update group.
Configuring BGP
Nonstop Routing
BGP Nonstop Routing (BGP NSR)
is enabled by default. The
no nsr disable command can also be used to
turn BGP NSR back on if it has been disabled.
Disable BGP Nonstop
Routing
Perform this task to disable BGP Nonstop Routing (NSR):
SUMMARY STEPS
configure
router bgp
as-number
nsr disable
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120
Specifies the BGP AS number, and enters the BGP configuration
mode, for configuring BGP routing processes.
Step 3
nsr disable
Example:
RP/0/RP0/CPU0:router(config-bgp)# nsr disable
Disables BGP Nonstop routing.
Step 4
commit
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:
SUMMARY STEPS
configure
router bgp
as-number
no nsr
disable
commit
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Step 2
router bgp
as-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 120
Specifies the
BGP AS number, and enters the BGP configuration mode, for configuring BGP
routing processes.
Step 3
no nsr
disable
Example:
RP/0/RP0/CPU0:router(config-bgp)# nsr disable
Enables BGP
Nonstop routing.
Step 4
commit
Configuring BGP
Additional Paths
Perform these tasks
to configure BGP Additional Paths capability:
Configures
additional paths selection capability for a prefix.
Step 11
commit
Originating 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.
Allows the redistribution of AiBGP metric into OSPF.
Step 8
commit
Configuring VRF
Dynamic Route Leaking
Perform these steps
to import routes from default-VRF to non-default VRF or to import routes from
non-default VRF to default VRF.
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.
SUMMARY STEPS
configure
vrfvrf_name
address-family{ipv4 |
ipv6}unicast
Use one of these
options:
import from default-vrfroute-policyroute-policy-name[ advertise-as-vpn]
export to default-vrfroute-policyroute-policy-name
import from default-vrfroute-policyroute-policy-name[ advertise-as-vpn]
export to default-vrfroute-policyroute-policy-name
Example:
RP/0/RP0/CPU0:router(config-vrf-af)#import from default-vrf route-policy rpl_dynamic_route_import
or
RP/0/RP0/CPU0:router(config-vrf-af)#export to default-vrf route-policy rpl_dynamic_route_export
Imports routes
from default-VRF to non-default VRF or from non-default VRF to default-VRF.
import from default-vrf—configures import from
default-VRF to non-default-VRF.
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.
export to default-vrf—configures import from
non-default-VRF to default VRF. The paths imported from the default-VRF are
advertised to other PEs.
Step 5
commit
What to do next
These
showbgp command
output displays information from the dynamic route leaking configuration:
Use the
showbgpprefix 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
showbgpimported-routes command to display IPv4 unicast
and IPv6 unicast address-families under the default-VRF.
Configuration Examples for Implementing BGP
This section provides the following configuration examples:
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
Displaying BGP
Update Groups: Example
The following is
sample output from the
show bgp
update-group command run in
XR 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
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
131.
108.0.0 and 192.
31.7.0 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
131.
108.234.2; and the third
neighbor and
remote-as
commands specify a neighbor on a different autonomous system.
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.
69.232.55 and 171.
69.232.56 get the local
preference, next hop, and MED unmodified in the updates. The router at
160.
69.69.1 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.
In a BGP speaker in
autonomous system 6002, the peers from autonomous systems 6001 and 6003 are
configured as special eBGP peers. Peer
170.
70.70.1 is a normal iBGP
peer, and peer 199.99.99.2 is a normal eBGP peer from autonomous system 700.
In a BGP speaker in
autonomous system 6003, the peers from autonomous systems 6001 and 6002 are
configured as special eBGP peers. Peer
200.
200.200.200 is a
normal eBGP peer from autonomous system 701.
The following is a
part of the configuration from the BGP speaker
200.
200.200.205 from
autonomous system 701 in the same example. Neighbor 171.
.232.56 is configured
as a normal eBGP speaker from autonomous system 666. The internal division of
the autonomous system into multiple autonomous systems is not known to the
peers external to the confederation.
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
VRF Dynamic Route Leaking Configuration: Example
These examples show how to configure VRF dynamic route leaking:
Import Routes from default-VRF to non-default-VRF
vrf vrf_1
address-family ipv6 unicast
import from default-vrf route-policy rpl_dynamic_route_import
!
end
Import Routes from non-default-VRF to default-VRF
vrf vrf_1
address-family ipv6 unicast
export to default-vrf route-policy rpl_dynamic_route_export
!
end
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.
Where to Go Next
For detailed information about BGP commands, see
Routing Command Reference for Cisco NCS 6000 Series Routers
Additional
References
The following
sections provide references related to implementing BGP.
IP Addresses and Services Command Reference for Cisco NCS 6000 Series Routers
Bidirectional Forwarding Detection (BFD)
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
Task ID
information.
Configuring AAA Services on
module of
System Security Configuration Guide for Cisco NCS 6000 Series Routers
Standards
Standards
Title
draft-bonica-tcp-auth-05.txt
Authentication for TCP-based Routing and Management
Protocols, by R. Bonica, B. Weis, S. Viswanathan, A. Lange, O. Wheeler
draft-ietf-idr-bgp4-26.txt
A Border
Gateway Protocol 4, by Y. Rekhter, T.Li, S. Hares
draft-ietf-idr-bgp4-mib-15.txt
Definitions of Managed Objects for the Fourth Version of Border
Gateway Protocol (BGP-4), by J. Hass and S. Hares
draft-ietf-idr-cease-subcode-05.txt
Subcodes
for BGP Cease Notification Message, by Enke Chen, V. Gillet
draft-ietf-idr-avoid-transition-00.txt
Avoid
BGP Best Path Transitions from One External to Another, by Enke Chen,
Srihari Sangli
draft-ietf-idr-as4bytes-12.txt
BGP
Support for Four-octet AS Number Space, by Quaizar Vohra, Enke Chen
Protection of BGP Sessions via the TCP MD5 Signature Option
RFC 2439
BGP
Route Flap Damping
RFC 2545
Use of
BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing
RFC 2796
BGP
Route Reflection - An Alternative to Full Mesh IBGP
RFC 2858
Multiprotocol Extensions for BGP-4
RFC 2918
Route
Refresh Capability for BGP-4
RFC 3065
Autonomous System Confederations for BGP
RFC 3392
Capabilities Advertisement with BGP-4
RFC 4271
A
Border Gateway Protocol 4 (BGP-4)
RFC 4724
Graceful Restart Mechanism for BGP
Technical
Assistance
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