Cisco ASR 9000 Series Routing Configuration Manual page 39

Implementing BGP
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 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
page
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
There is currently no way to control which prefixes are deleted.
For detailed configuration steps, see
Restrictions
These restrictions apply to the discard extra paths feature:
10.
deleted to match the new configured state value.
Cisco ASR 9000 Series Aggregation Services Router Routing Configuration Guide, Release 5.3.x
Configuring Discard Extra Paths, on page
BGP Maximum Prefix - Discard Extra Paths
BGP Default Limits, on
92.
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