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Filtering AS Paths with a Route Map

You can use a route map instead of the neighbor filter-list command to apply access lists for filtering routes. In Figure 25, suppose router Chicago is configured as follows:



host1(config)#router bgp 293 

host1(config-router)#network 192.168.5.0 mask
255.255.255.0 

host1(config-router)#neighbor 10.2.2.4 remote-as
17 

host1(config-router)#neighbor 10.2.2.4 weight
150 

host1(config-router)#neighbor 10.5.5.2 remote-as
32 

host1(config-router)#neighbor 10.5.5.2 weight
50

Figure 25: Route Map Filtering

Image g013181.gif

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:

Router Chicago prefers routes learned from router Boston (weight 150) over routes learned from router NY (weight 50), except that
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).

Refer to the commands and guidelines in the section Types of BGP Route Maps for more information about configuring route maps.

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