Setup Guide

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, see Access Control Lists (ACLs).
3. Return to CONFIGURATION mode.
CONFIG-ROUTE-MAP mode
exit
4. Enter ROUTER BGP mode.
CONFIGURATION mode
router bgp as-number
5. Filter routes based on the criteria in the configured route map.
CONFIG-ROUTER-BGP mode
neighbor {ip-address | ipv6-address | peer-group-name} route-map map-name {in | out}
Configure the following parameters:
● ip-address or ipv6-address or peer-group-name: enter the neighbor’s IPv4 or IPv6 address or the peer group’s
name.
● map-name: enter the name of a configured route map.
● in: apply the route map to inbound routes.
● out: apply the route map to outbound routes.
To view the BGP configuration, use the show config command in CONFIGURATION ROUTER BGP mode. To view a route
map configuration, use the show route-map command in EXEC Privilege mode.
Filtering on an AS-Path Attribute
You can use the BGP attribute, AS_PATH, to manipulate routing policies.
The AS_PATH attribute contains a sequence of AS numbers representing the route’s path. As the route traverses an AS, the
ASN is prepended to the route. You can manipulate routes based on their AS_PATH to affect interdomain routing. By identifying
certain ASN in the AS_PATH, you can permit or deny routes based on the number in its AS_PATH.
AS-PATH ACLs use regular expressions to search AS_PATH values. AS-PATH ACLs have an “implicit deny.” This means that
routes that do not meet a deny or match filter are dropped.
To configure an AS-PATH ACL to filter a specific AS_PATH value, use these commands in the following sequence.
1. Assign a name to a AS-PATH ACL and enter AS-PATH ACL mode.
CONFIGURATION mode
ip as-path access-list as-path-name
2. Enter the parameter to match BGP AS-PATH for filtering.
CONFIG-AS-PATH mode
{deny | permit} filter parameter
This is the filter that is used to match the AS-path. The entries can be any format, letters, numbers, or regular expressions.
You can enter this command multiple times if multiple filters are desired.
For accepted expressions, refer to Regular Expressions as Filters.
3. Return to CONFIGURATION mode.
AS-PATH ACL mode
exit
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Border Gateway Protocol (BGP)