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Configuration Example 1

Consider the network structure in Figure 3.

Suppose you want router London to behave in the following way:

Accept routes originated in AS 621 only if they pass directly to router London.
Accept routes originated in AS 11 only if they pass directly to router London.
Forward routes from AS 282 to AS 435 only if they pass through either AS 621 or AS 11, but not both AS 621 and AS 11.
Figure 3: Filtering with AS-Path Access Lists

The following commands configure router London to apply filters based on AS path to routes received from router Berlin and router Paris and to routes forwarded to router Madrid.



host1(config)#router bgp 47 

host1(config-router)#neighbor 10.2.9.2 remote-as
621 

host1(config-router)#neighbor 10.2.9.2 filter-list
1 in 

host1(config-router)#neighbor 10.2.8.2 remote-as
11 

host1(config-router)#neighbor 10.2.8.2 filter-list
2 in 

host1(config-router)#neighbor 10.2.7.2 remote-as
435 

host1(config-router)#neighbor 10.2.7.2 filter-list
3 out 

host1(config-router)#exit 

host1(config)#ip as-path access-list 1 deny ^11 

host1(config)#ip as-path access-list 1 permit .* 

host1(config)#ip as-path access-list 2 deny ^621 

host1(config)#ip as-path access-list 2 permit .* 

host1(config)#ip as-path access-list 3 deny [621 11] 

host1(config)#ip as-path access-list 3 permit .*

AS-path access list 1 is applied to routes that router London receives from router Paris. Router London rejects routes with the AS path 11 621 or 11 282 621.

AS-path access list 2 is applied to routes that router London receives from router Berlin. Router London rejects routes with the AS path 621 11 or 621 282 11.

Router London accepts routes with the AS path 282 11, 282 621, 282 621 11, or 282 11 621. However, it applies AS-path access list 3 to routes it forwards to router Madrid, and filters out routes with the AS path 282 621 11 or 282 11 621.

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