BGP AS Path Access Control List Filtering

Selecting routes from a BGP neighbour by using the AS path requires an AS Path Access Control List. An AS Patch Access Control List is very different from a standard or extended access control list.

An AS Path ACL utilises regular expressions as part of their filtering of routes based on the neighbours AS Path for routes. A Cisco router can have up to 500 AS Path ACLs

Processing of an AS Path ACL is carried out from a top to bottom order, with a permit or deny action on accepting the route or not. There is an implicit deny at the end of the AS Path ACL.

An example of an AS Path ACL is one that only permits locally originated routes:

! Only permit locally originated routes
ip as-path access-list 1 permit ^$

router bgp 65320
 address-family ipv4 unicast
 ! Only allowed locally originated routes advertised outbound
 neighbour 192.168.1.4 filter-list 1 out