9.2
Route Filtering and Manipulation Process (Policies)
9.2.7 Configuration example: communities
This example shows how the community attribute can be used to dynamically influence the routing decisions of another AS. With the network illustrated in the figure, the following configuration example shows how AS3 can advertise route 172.16.65.0/26 to AS1 and dynamically instruct AS1 not to advertise this route externally. AS3 will assign route 172.16.65.0/26 the community attribute "no-export" when advertising it to AS1.

Note: The send-community option in the neighbor router subcommand is used to cause the assigned community to be sent out.

Note: Click on topology to view command outputs.

The RTA configuration shows that RTA has defined a route map SETCOMMUNITY toward neighbor 172.16.20.1 (RTC). Clause 10 of the route map will match on prefix 172.16.65.0/26 and will set its community attribute to no-export. The send-community keyword assigned to the neighbor session is required to enable the community attribute to be sent to the specified neighbor. Clause 20 of the route map will enable all other networks to be passed with no change.

Note how the entry has been assigned the community no-export and instructions that it is not to be advertised to EBGP peers. RTC will not propagate this entry to its external peer RTD. Note that in the following RTD BGP table, RTD did not receive an update about 172.16.65.0/26.

Lab Activity
  In this lab, you will learn how to use the Community Attribute to dynamically influence the routing decisions of another AS.