Understanding the AS-Path Attribute

The AS-path attribute is a list of the ASs through which a route has passed. Whenever a route enters an AS, BGP prepends the AS number to the AS-path attribute. This feature enables network operators to track routes, but it also enables the detection and prevention of routing loops.

Consider the following sequence of events for the routers shown in Figure 34:

1. Route 172.21.10.0/23 is injected into BGP by means of router London in AS 47.
2. Suppose router London advertises that route to router Paris in AS 621. As received by router Paris, the AS-path attribute for route 172.21.10.0/23 is 47.
3. Router Paris advertises the route to router Berlin in AS 11. As received by router Berlin, the AS-path attribute for route 172.21.10.0/23 is 621 47.
4. Router Berlin advertises the route to router London in AS 47. As received by router London, the AS-path attribute for route 172.21.10.0/23 is 11 621 47.

   Figure 34: AS-Path Attributes

   

A routing loop exists if router London accepts the route from router Berlin. Router London can choose not to accept the route from router Berlin because it recognizes from the AS-path attribute (11 621 47) that the route originated in its own AS 47.

As a matter of routing policy, you can prepend additional AS numbers to the AS-path attribute for a route with a set as-path prepend clause in an outbound route map. Changing the AS path enables you to influence which of several routes for the same destination prefix is selected as the best route.

Copyright © 2008, Juniper Networks, Inc. All rights reserved. Trademark Notice.