BGP Engineering – Communities

• RFC 1998
• Pre-stages inbound policy to
  • match on communities
  • set local pref
  • set other attributes
• Results in outbound path selection.

The below routers are running in the following ASNs:

R1, R4 = ASN 200
R3, R2 = ASN 100

R4 is advertising its Loopback subnet of 4.4.4.0/24 into BGP. We’re going to change the Local Pref of the route 4.4.4.0 on R2 by using a community.

Configuration:

On R2 the configuration needs to first have communities setup:

These communities then need to be referenced in a Route Map, this one called ‘BGP_IN’.

This then needs to be applied to a neighbor statement. In this case with R1.

Next, we’ll add a prefix-list for R4’s loopback subnet.

Then a route-map called ‘BGP_OUT’.

This route-map will reference the Loopback subnet on R4 and set the community string to 100:110, which makes sense to R2.

Then the route-map will get applied to the neighbor statement towards R2.

Now on R2 we’re seeing that the local preference has changed in accordance with the route-map that sets values on communities.