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BGP Overview Configuring BGP
page 4-8 OmniSwitch 6800/6850/9000 Advanced Routing Configuration Guide December 2007
Communities
A community is a group of destinations that share some common property. A community is not restricted
to one network or one autonomous system.
Communities are used to simplify routing policies by identifying routes based on a logical property rather
than an IP prefix or an AS number. A BGP speaker can use this attribute in conjunction with other
attributes to control which routes to accept, prefer, and pass on to other BGP neighbors.
Communities are not limited by physical boundaries, and routers in a community can belong to different
ASs.
For example, a community attribute of “no export” could be added to a route, preventing it from being
exported, as shown:
In the above example, Route A is not propagated to AS 100 because it belongs to a community that is not
to be exported by a speaker that learns it.
A route can have more than community attribute. A BGP speaker that sees multiple community attributes
in a route can act on one, several, or all of the attributes. Community attributes can be added or modified
by a speaker before being passed on to other peers.
Communities are discussed further in “Working with Communities” on page 4-43.
Route A
(No Export)
Route B
Route B
AS 100 AS 200 AS 300