Administrator Guide

To filter routes using prefix lists, use the following commands.
1. Create a prefix list and assign it a name.
CONFIGURATION mode
ip prefix-list prefix-name
2. Create multiple prefix list filters with a deny or permit action.
CONFIG-PREFIX LIST mode
seq sequence-number {deny | permit} {any | ip-prefix [ge | le] }
● ge: minimum prefix length to be matched.
● le: maximum prefix length to me matched.
For information about configuring prefix lists, refer to Access Control Lists (ACLs).
3. Return to CONFIGURATION mode.
CONFIG-PREFIX LIST mode
exit
4. Enter ROUTER BGP mode.
CONFIGURATION mode
router bgp as-number
5. Filter routes based on the criteria in the configured prefix list.
CONFIG-ROUTER-BGP mode
neighbor {ip-address | peer-group-name} distribute-list prefix-list-name {in | out}
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.
As a reminder, the following are 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).
● After 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 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.
Filtering BGP Routes Using Route Maps
To filter routes using a route map, use these commands.
1. Create a route map and assign it a name.
CONFIGURATION mode
route-map map-name [permit | deny] [sequence-number]
2. Create multiple route map filters with a match or set action.
CONFIG-ROUTE-MAP mode
{match | set}
For information about configuring route maps, refer to Access Control Lists (ACLs).
3. Return to CONFIGURATION mode.
CONFIG-ROUTE-MAP mode
exit
4. Enter ROUTER BGP mode.
CONFIGURATION mode
200
Border Gateway Protocol IPv4 (BGPv4)