Reference Guide

210 | Border Gateway Protocol
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Prior to filtering BGP routes, you must create the prefix list, AS-PATH ACL, or route map to be used.
Refer to Chapter 6, “Access Control Lists (ACLs),” on page 87 for configuration information on prefix
lists, AS-PATH ACLs, and route maps.
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.
Note: When you configure a new set of BGP policies, always reset the neighbor or peer group by
entering the
clear ip bgp command in EXEC Privilege mode.
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 Chapter 6, “Access Control Lists
(ACLs),” on page 87 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.