You can filter updates received from BGP4 neighbors based on the contents of the AS-path list
accompanying the updates. For example, if you want to deny routes that have the AS 4.3.2.1 in the
AS-path from entering the BGP4 route table, you can define a filter to deny such routes.

The device provides the following methods for filtering on AS-path information:

•

AS-path filters - refer to

“Defining an AS-path filter”

on page 828.

•

AS-path ACLs

NOTE

The BigIron RX cannot actively support AS-path filters and AS-path ACLs at the same time. Use one
method or the other but do not mix methods.

NOTE

Once you define a filter or ACL, the default action for updates that do not match a filter is “deny”. To
change the default action to “permit”, configure the last filter or ACL as “permit any any”.

AS-path filters or AS-path ACLs can be referred to by a BGP neighbor's filter list number as well as
by match statements in a route map.

Defining an AS-path ACL

To configure an AS-path list that uses ACL 1, enter a command such as the following.

BigIron RX(config)# ip as-path access-list acl1 permit 100

BigIron RX(config)# router bgp

BigIron RX(config-bgp)# neighbor 10.10.10.1 filter-list 1 in

The ip as-path command configures an AS-path ACL that permits routes containing AS number 100
in their AS paths. The neighbor command then applies the AS-path ACL to advertisements and
updates received from neighbor 10.10.10.1. In this example, the only routes the device permits
from neighbor 10.10.10.1 are those whose AS-paths contain AS-path number 100.

Syntax: ip as-path access-list <string> [seq <seq-value>] deny | permit <regular-expression>

The <string> parameter specifies the ACL name. (If you enter a number, the CLI interprets the
number as a text string.)