OS10 Enterprise Edition User Guide Release 10.4.0E(R3)

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AS path and next-hop

The AS path is the AS list that all the prefixes listed in the update have passed through. The BGP speaker adds the local AS number when advertising to an EBGP neighbor. Any update that contains the AS path number 0 is valid.

The next-hop is the IP address used to reach the advertising router:
  • For EBGP neighbors, the next-hop address is the IP address of the connection between neighbors.
  • For IBGP neighbors, the EBGP next-hop address is carried into the local AS. A next hop attribute sets when a BGP speaker advertises itself to another BGP speaker outside the local AS and when advertising routes within an AS.

For EBGP neighbors, the next-hop address corresponding to a BGP route does not resolve if the next-hop address is not the same as the neighbor IP address. The next-hop attribute also serves as a way to direct traffic to another BGP speaker, instead of waiting for a speaker to advertise. When a next-hop BGP neighbor is unreachable, the connection to that BGP neighbor goes down after the hold-down timer expiry.

When you enable fast-external-fallover and if the router has learned the routes from the BGP neighbor, the BGP session terminates immediately if the next-hop becomes unreachable—without waiting for the hold-down time.


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