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This chapter is from the book

Filtering BGP Routes

You can combine route maps with prefix lists to filter the routes advertised to or received from a BGP peer, to control routes redistributed into BGP, and to set BGP attributes for specific routes. Prefix lists alone can be applied to a neighbor to filter route updates.

To use a prefix list, plan the implementation by determining the requirements. Then create a prefix list to match the networks to be filtered. Permit the networks you want to allow to be advertised and deny all others. Next apply the prefix list to the BGP neighbor, inbound or outbound. The next example shows a prefix list that permits only summary routes in the 172.31.0.0 network. All other routes are denied by default. The prefix list is then applied to BGP neighbor 10.1.1.1 outbound, so only these routes will be advertised to that peer:

ip prefix-list Summary permit 172.31.0.0/16 le 20
!
router bgp 65001
neighbor 10.1.1.1 prefix-list Summary out

To verify the results of your configuration use the command show ip prefix-list. To clear the counters shown in that command, use the clear ip prefix-list command.

Combine a prefix list with a route map to set attributes on the routes allowed in the prefix list. In the following example, prefix list Summary is used again. A route map sets the Med for those routes to 100 when they are advertised. It sets a Med of 200 for all other routes advertised. The route map is then applied to BGP neighbor 10.1.1.1 outbound:

route-map CCNP permit 10
match ip address prefix-list Summary
set metric 100
route-map CCNP permit 20
set metric 200
!
router bgp 65001
neighbor 10.1.1.1 route-map CCNP out
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