Juniper BGP, Configuration Manual

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Figure 23: Filtering with AS-Path Access Lists
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
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.
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 .*
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
JUNOSe 11.1.x BGP and MPLS Configuration Guide