The customer is assigned the private AS number 65001 by the ISP; therefore, routes that are received by the provider have an AS path containing only AS 65001. This information should be kept and used within the ISP network and should never be propagated to the rest of the Internet (AS 217 in this example).
The edge router in AS 387 has been configured to remove private AS numbers on EBGP routes toward AS 217. If private AS numbers appear in the tail end of the AS path (before AS 387 is added), they are removed.
This configuration must be applied to all egress routers in AS 387 that serve EBGP neighbors leading to other ISPs. No private AS number may be present in an AS path of a route that is propagated to a network using a public AS number.
© 2005, Cisco Systems, Inc. Customer-to-Provider Connectivity with BGP 5-59
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