System information

Configuring Routing Policy
7-9
out permits the outgoing packet to be processed only if access-list permits the
packet.
Creating Community Lists
You can use the community to control the routing information a BGP speaker accepts,
prefers, or distributes to other neighbors. The BGP community attribute passes
between peers when they exchange reachability information. You can use the
following predefined community attribute keywords with the set community
command in a route map:
• no-export
• no-advertise
• local-as
Use the no export keyword to disallow advertising to EBGP peers. This is useful in a
network that uses IBGP heavily but does not want to share its internal routing policies
with its EBGP peers. Use the no-advertise keyword to prevent communities from
being propagated beyond the local router, even to IBGP peers.
Figure 7-1 shows how you can create a route map based on the network shown. The
Router Boston sets the value of the local preference attribute based on the value of the
community attribute. Any route that has a community attribute of 100 matches
community list 1 and has its local preference set to 50. Any route that has a
community attribute of 200 matches community list 2 and has its local preference set
to 25. All other routes do not have their local preference attributes changed, because
all routes are members of the Internet community. All destinations belong to the
general Internet community by default.