Service Manual

Filtering BGP Routes Using AS-PATH
Information
To filter routes based on AS-PATH information, use these commands.
1 Create a AS-PATH ACL and assign it a name.
CONFIGURATION mode
ip as-path access-list as-path-name
2 Create a AS-PATH ACL filter with a deny or permit action.
AS-PATH ACL mode
{deny | permit} as-regular-expression
3 Return to CONFIGURATION mode.
AS-PATH ACL
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 | peer-group-name} filter-list as-path-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.
• as-path-name: enter the name of a configured AS-PATH ACL.
• in: apply the AS-PATH ACL map to inbound routes.
• out: apply the AS-PATH ACL to outbound routes.
To view which commands are configured, use the show config command in CONFIGURATION ROUTER
BGP mode and the
show ip as-path-access-list command in EXEC Privilege mode.
To forward all routes not meeting the AS-PATH ACL criteria, include the permit .* filter in your AS-PATH ACL.
Filtering BGP Routes
Filtering routes allows you to implement BGP policies.
You can use either IP prefix 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. Prefix lists filter routes based on
route and prefix length, while AS-Path ACLs filter routes based on the ASN. Route maps can filter and set
conditions, change attributes, and assign update policies.
Border Gateway Protocol IPv4 (BGPv4) 253