Concept Guide

Filtering BGP Routes
Filtering routes allows you to implement BGP policies.
You can use either IP prex lists, route maps, AS-PATH ACLs or IP community lists (using a route map) to control which routes the BGP
neighbor or peer group accepts and advertises. Prex lists lter routes based on route and prex length, while AS-Path ACLs lter routes
based on the ASN. Route maps can lter and set conditions, change attributes, and assign update policies.
NOTE: The system supports up to 255 characters in a set community statement inside a route map.
NOTE: You can create inbound and outbound policies. Each of the commands used for ltering has in and out parameters that
you must apply. The order of preference varies depending on whether the attributes are applied for inbound updates or outbound
updates.
For inbound and outbound updates the order of preference is:
• prex lists (using the neighbor distribute-list command)
• AS-PATH ACLs (using the neighbor filter-list command)
• route maps (using the neighbor route-map command)
Prior to ltering BGP routes, create the prex list, AS-PATH ACL, or route map.
For conguration information about prex lists, AS-PATH ACLs, and route maps, refer to Access Control Lists (ACLs).
NOTE: When you congure a new set of BGP policies, to ensure the changes are made, always reset the neighbor or peer group
by using the clear ip bgp command in EXEC Privilege mode.
To lter routes using prex lists, use the following commands.
1 Create a prex list and assign it a name.
CONFIGURATION mode
ip prefix-list prefix-name
2 Create multiple prex list lters with a deny or permit action.
CONFIG-PREFIX LIST mode
seq sequence-number {deny | permit} {any | ip-prefix [ge | le] }
• ge: minimum prex length to be matched.
• le: maximum prex length to me matched.
For information about conguring prex 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 congured prex list.
CONFIG-ROUTER-BGP mode
neighbor {ip-address | peer-group-name} distribute-list prefix-list-name {in | out}
Congure the following parameters:
216
Border Gateway Protocol IPv4 (BGPv4)