Reference Guide

Border Gateway Protocol | 157
Regular Expressions as filters
Regular expressions are used to filter AS paths or community lists. A regular expression is a special
character used to define a pattern that is then compared with an input string.
For an AS-path access list as shown in the commands above, if the AS path matches the regular expression
in the access list, then the route matches the access list.
Figure 8-27 applies access list Eagle to routes inbound from BGP peer 10.5.5.2. Access list Eagle uses a
regular expression to deny routes originating in AS 32.
Figure 8-27. Filtering with Regular Expression
FTOS(config)#router bgp 99
FTOS(conf-router_bgp)#neigh AAA peer-group
FTOS(conf-router_bgp)#neigh AAA no shut
FTOS(conf-router_bgp)#show conf
!
router bgp 99
neighbor AAA peer-group
neighbor AAA no shutdown
neighbor 10.155.15.2 remote-as 32
neighbor 10.155.15.2 shutdown
FTOS(conf-router_bgp)#neigh 10.155.15.2 filter-list 1 in
FTOS(conf-router_bgp)#ex
FTOS(conf)#ip as-path access-list Eagle
FTOS(config-as-path)#deny 32$
FTOS(config-as-path)#ex
FTOS(conf)#router bgp 99
FTOS(conf-router_bgp)#neighbor AAA filter-list Eagle in
FTOS(conf-router_bgp)#show conf
!
router bgp 99
neighbor AAA peer-group
neighbor AAA filter-list Eaglein
neighbor AAA no shutdown
neighbor 10.155.15.2 remote-as 32
neighbor 10.155.15.2 filter-list 1 in
neighbor 10.155.15.2 shutdown
Create the Access List and Filter
Regular Expression shown
as part of Access List filter