Cisco ASR 9000 Series Routing Configuration Guide

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.
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).
Note
BGP Maximum Prefix - Discard Extra Paths
IOS XR BGP maximum-prefix feature imposes a maximum limit on the number of prefixes that are received
from a neighbor for a given address family. Whenever the number of prefixes received exceeds the maximum
number configured, the BGP session is terminated, which is the default behavior, after sending a cease
notification to the neighbor. The session is down until a manual clear is performed by the user. The session
can be resumed by using the clear bgp command. It is possible to configure a period after which the session
can be automatically brought up by using the maximum-prefix command with the restart keyword. The
maximum prefix limit can be configured by the user. Default limits are used if the user does not configure
the maximum number of prefixes for the address family. For default limits, refer to BGP Default Limits, on
page 10.
Discard Extra Paths
An option to discard extra paths is added to the maximum-prefix configuration. Configuring the discard extra
paths option drops all excess prefixes received from the neighbor when the prefixes exceed the configured
maximum value. This drop does not, however, result in session flap.
The benefits of discard extra paths option are:
•
Limits the memory footstamp of BGP.
•
Stops the flapping of the peer if the paths exceed the set limit.
When the discard extra paths configuration is removed, BGP sends a route-refresh message to the neighbor
if it supports the refresh capability; otherwise the session is flapped.
On the same lines, the following describes the actions when the maximum prefix value is changed:
•
If the maximum value alone is changed, a route-refresh message is sourced, if applicable.
•
If the new maximum value is greater than the current prefix count state, the new prefix states are saved.
•
If the new maximum value is less than the current prefix count state, then some existing prefixes are
deleted to match the new configured state value.
There is currently no way to control which prefixes are deleted.
For detailed configuration steps, see Configuring Discard Extra Paths, on page 92.
Restrictions
These restrictions apply to the discard extra paths feature:
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
9
Implementing BGP
BGP Maximum Prefix - Discard Extra Paths

Cisco ASR 9000 Series Specifications

General

Brand	Cisco
Model	ASR 9000 Series
Category	Network Router
Language	English