Reference Guide

Border Gateway Protocol IPv4 (BGPv4) | 239
Use these commands in the following sequence, starting in the CONFIGURATION mode to filter routes
using prefix lists.
As a reminder, below are some rules concerning prefix lists:
• If the prefix list contains no filters, all routes are permitted.
• If none of the routes match any of the filters in the prefix list, the route is denied. This action is called
an implicit deny. (If you want to forward all routes that do not match the prefix list criteria, you must
configure a prefix list filter to permit all routes. For example, you could have the following filter as the
last filter in your prefix list
permit 0.0.0.0/0 le 32).
• Once a route matches a filter, the filter’s action is applied. No additional filters are applied to the route.
To view the BGP configuration, use the
show config command in the ROUTER BGP mode. To view a
prefix list configuration, use the
show ip prefix-list detail or show ip prefix-list summary commands in
EXEC Privilege mode.
Use these commands in the following sequence, starting in the CONFIGURATION mode to filter routes
using a route map.
Step Command Syntax Command Mode Purpose
1
ip prefix-list prefix-name CONFIGURATION Create a prefix list and assign it a name.
2
seq sequence-number {deny |
permit} {any | ip-prefix [ge | le]}
CONFIG-PREFIX LIST Create multiple prefix list filters with a deny
or permit action.
ge: Minimum prefix length to be matched
le: maximum prefix length to me matched
Refer to the IP Access Control Lists (ACL),
Prefix Lists, and Route-maps chapter for
information on configuring prefix lists.
3
exit
CONFIG-PREFIX LIST Return to the CONFIGURATION mode.
4
router bgp as-number CONFIGURATION Enter ROUTER BGP mode.
5
neighbor {ip-address |
peer-group-name}
distribute-list prefix-list-name
{in | out}
CONFIG-ROUTER-BGP Filter routes based on the criteria in the
configured prefix list. Configure the
following parameters:
• ip-address or peer-group-name: enter
the neighbor’s IP address or the peer
group’s name.
• prefix-list-name: enter the name of a
configured prefix list.
• in: apply the prefix list to inbound routes.
• out: apply the prefix list to outbound
routes.
Step Command Syntax Command Mode Purpose
1 route-map map-name [permit |
deny] [sequence-number]
CONFIGURATION Create a route map and assign it a name.