Bgp Router Identifier; Bgp Default Limits - Cisco NCS 5500 Series Configuration Manual

Information about Implementing BGP

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.
We strongly recommend that the bgp router-id command is configured to prevent unnecessary changes
Note
to the router ID (and consequent flapping of BGP sessions).

BGP Default Limits

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:
BGP Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
118
◦ 512K (524,288) prefixes for IPv4 unicast
◦ 128K (131,072) prefixes for IPv6 unicast
◦ 512K (524,288) prefixes for VPNv4 unicast
Implementing BGP