9.3
The Community Attribute
9.3.2 Community attribute configuration example
As a further example, consider the network shown in Figure . You can configure RTA to send the NO_EXPORT Community attribute when updating RTC about the route to 172.16.65.0/24.

To prevent AS2 from learning the 172.16.65.0/24 route from AS1, you can configure RTA (AS3) according to configuration shown in the figure (click on the topology to view the command output).

The command output in the example shows that RTA has defined a route map SETCOMMUNITY and will send that value toward neighbor 172.16.20.1 (RTC). Clause 10 of the route map will match on prefix 172.16.65.0/24 and will set its Community attribute to NO_EXPORT. Clause 20 of the route map will enable all other networks to be passed with no change.

Notice that RTA is configured with the send-community option in the neighbor statement. This option is necessary to instruct RTA to send the assigned community value out to that neighbor.

Although the NO_EXPORT and NO_ADVERTISE values are common, autonomous systems can also agree on private Community attributes to implement routing policy. For example, RTX in AS 100 can be configured to set the community attribute for certain routes to the private value of 50, as shown in Figure .

In Figure , RTX is configured to match BGP routes according to access list 1 and to set the Community attribute to 50 in outbound updates. The keyword additive (included in the route map) instructs RTX to append the value 50 to the existing community values. In this way, multiple community values will form a list, allowing greater flexibility.

Assume RTX has a neighbor, RTZ(1.1.1.2). RTZ resides in a different AS and has established an EBGP session with RTX. The administrators of both systems have agreed on the significance of the community value, 50. Thus, RTZ is configured to react accordingly.

When configured according to Figure , RTZ will match routes that arrive with a community value of 50 and then will set the Local Preference attribute for those routes to 400.