Figure 25: Route Map Filtering
Routes learned from router Boston have a weight of 150, whereas those learned
from router NY have a weight of 50. Router Chicago therefore prefers all routes
learned from router Boston to those learned from router NY. Based on this
configuration, router Chicago prefers routes to prefixes originating in AS 837 or
originating in AS 32 that pass through router Boston over routes to those same
prefixes that pass through router NY.
This is a longer path than you might desire. You can avoid this result by configuring
a route map to modify the weight of certain routes learned by router Chicago:
host1(config-router)#neighbor 10.5.5.2 route-map alpha in
host1(config-router)#exit
host1(config)#route-map alpha permit 10
host1(config-route-map)#match as-path dog1
host1(config-route-map)#set weight 175
host1(config-route-map)#exit
host1(config)#ip as-path access-list dog1 permit _32$
host1(config)#ip as-path access-list dog1 permit _837$
host1(config)#route-map alpha permit 20
host1(config-route-map)#match as-path dog2
host1(config-route-map)#exit
host1(config)#ip as-path access-list dog2 permit .*
BGP applies route map alpha to all routes learned from 10.5.5.2 (router NY). Instance
10 of route map alpha matches routes with access list dog1. This access list permits
any route whose AS-path attribute ends in 32 or 837—that is, routes that originate
in AS 32 or AS 837. It sets their weight to 175, overriding the neighbor weight (50)
set for updates received from 10.5.5.2. Then, instance 20 of route map alpha permits
all other routes with no modification.
The result of this improved configuration is the following:
â– Router Chicago prefers routes learned from router Boston (weight 150) over
routes learned from router NY (weight 50), except that
â– Router Chicago prefers routes learned from router NY that originate in AS 837
or AS 32 (weight 175 as a result of route map alpha) over the same routes learned
from router Boston (weight 150).
Configuring BGP Routing Policy â– 91
Chapter 1: Configuring BGP Routing
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