AS PATH PADDING or PREPENDING is the process of stamping multiple instances of one's own AS to a route announcement to de-prefer that path for inbound traffic. Customers can use PATH PADDING to influence the routing behavior of external sources trying to reach the customer. PATH PADDING may not affect the directly connected network. In other words, traffic that originates on the AT&T IP Backbone will use the direct connection to reach the customer regardless of the prepending that has been done to that route announcement. This is because a directly connected customer has a higher local-preference (BGP attribute) than a peer route and local-preference is taken into account BEFORE AS PATH.
Try this link:
http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_9-1/autonomous_system_numbers.html
