Spath prepending is used to

• Ensure proper return path selection

• Distribute the return traffic load for multihomed customers

You can manipulate AS paths by prepending AS numbers to already 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.

Copyright © 2004, Cisco Systems, Inc. Route Selection Using Attributes 4-47

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