BGP Maximum-prefix neighbor question

[hfakoor222]

Hi, 

I am studying for the Ensari exam, part of the studies include the maximum-prefix feature received from a neighbor. When the session is torn down, after the prefix is reached, if warning-only (or potentially restart) keywords aren't used, well then the session is torn down...

My question is, are the prefixes being advertised, or is the session torn down and connectivity is lost?

Wouldn't it be better not to accept new prefixes at that point instead of tearing the session down (assuming the prefixes aren't being advertised?)

If the current prefixes stop being advertised, I don't understand the logic behind this.  The books and the documentation were a little vague for me on this point, if someone could clarify it for me.

[M02@rt37 · Accepted solution]

Hello @hfakoor222,

The maximum-prefix feature is used in BGp to limit the number of prefixes received from a neighbor. When the maximum number of prefixes is reached, the behavior of the BGP session depends on how the maximum-prefix feature is configured.

If the session is configured with the maximum-prefix feature without any additional keywords (such as warning-only or potentially restart), and the maximum number of prefixes is exceeded, the BGP session will be torn down. This means that the connectivity to that neighbor will be lost, and the session will need to be reestablished.

The purpose of tearing down the session in this scenario is to protect the router from potential routing instability or memory exhaustion caused by receiving a large number of prefixes. It serves as a mechanism to prevent excessive or unexpected prefix advertisement and to maintain the stability of the BGP routing system.

Instead of simply not accepting new prefixes when the maximum is reached, tearing down the session provides a more proactive approach. By tearing down the session, the router ensures that it avoids any potential issues that could arise from continuing to receive and process additional prefixes.

[MHM Cisco World]

I will share lab tonight testong this case.

[MHM Cisco World · Accepted solution]

first I want to inform you, about your previous post about EEM and multi interface I found solution but I need to test it first
second the BGP with max prefix and restart, the IOU2 restart the session and even after I hard refresh the BGP in IOU1 the IOU2 still end the session, the only way to re-session the BGP is in IOU2 (with max prefix config) by hard refresh BGP