3.7.d [iv] Communities, extended communities
- BGP communities are a means of tagging routes to ensure consistent filtering or route selection policy
- A community is an attribute that is used to tag BGP routes.
- There can be more than one BGP community that is attached to a single route, but the routers, by default, remove communities in outgoing BGP updates.
- The community attribute is a transitive optional attribute. Its value is a 32-bit number.
- The following is a set of community values that has been predefined:
- no-advertise – If a router receives an update carrying this community, it will not propagate it to any neighbor.
- no-export – If a router receives an update carrying this community, it will not propagate it to any external neighbors except intra-confederation external neighbors. This is the most widely used predefined community attribute
- local-as – This community has a similar meaning to “no-export”, but it keeps a route within the local AS (or member-AS within the confederation). The route is not sent to external BGP neighbors or to intra-confederation external neighbors.
- internet – Advertise this route to the Internet community. All routers belong to it.
- Routers that do not support the community attribute will pass the attribute to other neighbors because it is a transitive attribute.
- A 32-bit community value is split into two parts:
- High-order 16 bits contains the AS number of the AS that defines the community meaning
- Low-order 16 bits have local significance.
- Example: [AS-number]:[low-order-16-bits]
- Values of all zeros and all ones in high-order 16 bits are reserved