Reference Guide

228 | Border Gateway Protocol IPv4 (BGPv4)
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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.
The following example 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.
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
FTOS(conf-router_bgp)#ex
FTOS(conf)#ex
FTOS#show ip as-path-access-lists
ip as-path access-list Eagle
deny 32$
FTOS#