Figure 23: Filtering With As-Path Access Lists - Juniper BGP - CONFIGURATION GUIDE V 11.1.X Configuration Manual

JUNOSe 11.1.x BGP and MPLS Configuration Guide

Figure 23: Filtering with AS-Path Access Lists

Suppose you want router London to behave in the following way:
The following commands configure router London to apply filters based on the AS
path to routes received from router Berlin and router Paris and to routes forwarded
to router Madrid.
AS-path access list 1 is applied to routes that router London receives from router
Paris. Router London rejects routes with the AS path (621 11).
AS-path access list 2 is applied to routes that router London receives from router
Berlin. Router London rejects routes with the AS path (11 621) or (621 282 11).
Router London accepts routes with the AS path (11 282), (621 282), (621 11 282),
or (11 621 282). However, it applies AS-path access list 3 to routes it forwards to
router Madrid, and filters out routes with the AS path (621 11 282) or (11 621 282).
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Configuring BGP Routing Policy
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
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 ^621_11$
host1(config)#ip as-path access-list 1 permit .*
host1(config)#ip as-path access-list 2 deny ^11_621$
host1(config)#ip as-path access-list 2 permit .*
host1(config)#ip as-path access-list 3 deny ^11_621_282
host1(config)#ip as-path access-list 3 deny ^621_11_282
host1(config)#ip as-path access-list 3 permit .*