Well Known BGP Communities

The following are few well-known communities that must be recognized by all community aware BGP implementations. Communities affect peer-to-peer routing policies. We tell the peer to handle prefixes differently by tagging them with communities.

1. No-Advertise Community: A BGP speaker won’t advertise a route to any internal or external BGP peers if a No-Advertise community is linked to it.

2. No-Export Community: ‍When a No-Export community is attached to a route, the router won’t advertise the route to external peers–only to internal peers.

3. Local AS Community: A crucial guideline for internal BGP neighbors is that they cannot advertise a route to another IBGP neighbor if they obtained it from another IBGP neighbor. This is by default in order to prevent BGP routing loops.

4. Graceful Shutdown Community: The Graceful_SHUTDOWN (65535:0) community is used to smoothly shut down paths a router might use when its peer router is about to be intentionally shut down.

5. Extended Community: An Extended community is a 8-byte value that is divided into two main sections: First 2 bytes: Specifies community type. Last 6 bytes: Provides unique information specific to the community type. The most well-known application for extended communities is MPLS-VPN, which employs two extended communities: Route Target community: Identifies the routers that can receive a specific set of routes. Route Origin community: Identifies the routers that inject a specific set of routes into BGP.

6. Route Target Community: The Route Target community is used in MPLS VPN environments to separate two customers routing tables

7. Route Origin Community: In an MPLS VPN environment, the route origin community is used to identify where routes originated from, so that readvertisement back to that site is avoided.

8. Large Communities: A Large community is a 12-byte BGP community that was developed when the 4-byte AS began to be allocated. Since each of the standard or extended communities use 2-byte values for the AS, a 4-byte AS would not fit into the standard 2-byte value.