Sunday, October 2, 2011

Deleting BGP Communities

BGP communities are passed between bgp peers when the 'send community' attribute is set with the neighbor. However what if there is a requirement to delete one of the communities from a list associated with a route!?

Consider the topology below. The route is advertised by R1 to R2 and the route has community attributes 'no-advertise' and 'internet'.    R2 has an iBGP peering with R3 and the send-community attribute is set.   Initially by default R3 does not receive the route as a direct result of the no-advertise community.  The requirement is for R3 to have the BGP route with only the internet attribute.

The solution relies on the 'set comm-list' command. This enables communities matched by the community-list to be removed.  Below i attach the working config from R2.


ip community-list standard RIC permit no-advertise

route-map RIC permit 10
 set comm-list RIC delete

router bgp 1
 neighbor remote-as 2
 neighbor route-map RIC in

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