BGP Maximum Prefix Feature

The BGP Maximum-Prefix feature allows you to control how many prefixes can be received from a BGP neighbor. By default, this feature allows a router to bring down a peer when the number of received prefixes from that peer exceeds the configured Maximum-Prefix limit. This feature is commonly used for eBGP peers, but can be applied to iBGP peers also.

The Maximum-Prefix feature is useful when, at a change of outbound policy at the remote peering site, a router starts to receive more routes than the router memory can take. If this same router is peering with BGP and also performs critical routing functions within a network, this overhead could cause the router to fail.

With the neighbor maximum-prefix command, it is possible to protect a router against this situation.

When you plan to use this feature, consider these key points:

• Know how many routes the remote BGP peering router normally sends.

• Set a threshold a little higher than the number of BGP prefixes expected to be received during normal operations.

• Know the action to take in case the remote BGP peer sends more prefixes than those expected. Available actions could either be to bring down the session and to keep the BGP neighbor relationship down until you use the clear ip bgp x.x.x.x command or, alternatively, to only log a warning message.