This topic describes the function of AS-path prepending and how you can use it to facilitate proper return path selection.
You can manipulate AS paths by prepending AS numbers to existing AS paths. Normally, you perform AS-path prepending on outgoing EBGP updates over the nondesired return path. Because the AS paths sent out over the nondesired link become longer than the AS path sent out over the preferred path, the nondesired link is now less likely to be used as the return path.
The length of the AS path is extended because additional copies of the AS number of the sender are prepended to (added to the beginning of) the AS-path attribute. To avoid clashes with BGP loop prevention mechanisms, no other AS number, except that of the sending AS, should be prepended to the AS-path attribute.
If another AS number is prepended in the AS path, the routers in the AS that has been prepended will reject the update because of BGP loop prevention mechanisms.
You can configure prepending on a router for all routing updates that you send to a neighbor or only on a subset of them.
4-42 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.
Example: AS-Path Prepending
In this example, administrative policy in AS 213 prefers that the low-speed link be used for backup purposes only.
AS-Path Prepending (Cont.)
As long as the high-speed link between AS 213 and AS 462 is available, all traffic should flow toward AS 213 using the high-speed link.
To accomplish this goal, you can configure the router in AS 213 that sends EBGP updates to AS 387 by prepending the AS path with two copies of the AS number 213. AS 387 receives two alternative routes to reach network 10.0.0.0/8: the update that it has received directly from AS 213 (that has a manipulated AS path with a length of three) and the update that it has received via AS 462 (that was not manually manipulated and therefore contains an AS-path length of two).
When AS 387 starts the route selection process to determine which route to use to reach network 10.0.0.0/8, it checks the AS-path length after the weight and local preference parameters. In this case, neither weight nor local preference has been configured, so the length of the AS path will be the deciding factor in the route selection process. Consequently, AS 387 prefers the shortest AS path and thus forwards packets toward network 10.0.0.0/8 via AS 462. The desired administrative policy has been met, and AS 213 will receive incoming traffic over the high-speed link.
If the forwarding path from AS 387 via AS 462 to AS 213 and network 10.0.0.0/8 is later broken, the BGP update to reach network 10.0.0.0/8 is revoked. In case of such a network failure, AS 387 will have only one remaining path to reach network 10.0.0.0/8. The route selection process now has only one choice, the route directly to AS 213 over the low-speed WAN link. The low-speed link will therefore serve as backup to the high-speed WAN link.
© 2005, Cisco Systems, Inc. Route Selection Using Attributes 4-43
AS-Path Prepending (Cont.)
When you are manually manipulating AS paths, the only valid AS number that you can prepend is the AS number of the sender. Prepending any other AS number will cause problems.
In the example, AS 213 is prepending AS number 387. The egress router performs AS-path prepending when the route is on its way to be transmitted to AS 387. After the manual manipulation, BGP automatically changes the AS path according to the BGP specifications. The local AS number should always be added first when updates are sent over an EBGP session. Therefore, when AS 387 receives the BGP update, the AS path contains the value 213 387. The AS number 387 was set by the manual manipulation, and the AS number 213 was prepended automatically by BGP because the update was sent over an EBGP session.
When the edge router in AS 387 receives the BGP update, it checks the AS path to verify that the BGP updates were not propagated accidentally by a routing loop. Because the edge router finds its own AS number in the AS path, it assumes that the BGP update has already been in AS 387. According to the BGP specification, the update will be silently ignored.
Now assume that AS 213 had, for the manual manipulation, used a different AS number, not its own and not AS number 387. Would AS 387 now have accepted the update? The answer is yes. However, in this scenario, a problem would have appeared at a later stage when the route finally reached the actual AS belonging to the manually prepended AS number. This AS would have rejected the route because it would have found its own AS number somewhere in the AS path.
4-44 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.
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