Dell SmartFabric OS10 User Guide Release 10.5.3

You can transparently change the AS number of an entire BGP network. Changing the AS number ensures that the routes propagate throughout the network while migration is in progress. When migrating one AS to another and combining multiple AS, an external BGP (eBGP) network may lose its routing to an internal BGP (iBGP) if the AS number changes.

Migration is difficult as all iBGP and eBGP peers of the migrating network must be updated to maintain network reachability. Local-AS allows the BGP speaker to operate as if it belongs to a virtual AS network besides its physical AS network.

Disable the local-as command after migration. Failure to disable the local-as command after migration causes the local-as command to replace the original AS number of the system. Reconfigure the system with a new AS number.

Router A, Router B, and Router C belong to AS 100, 200, and 300, respectively. Router A acquired Router B — Router B has Router C as its client. When Router B is migrating to Router A, it must maintain the connection with Router C without immediately updating Router C’s configuration. Local-AS allows Router B to appear as if it still belongs to Router B’s old network, AS 200, to communicate with Router C.

The Local-AS does not prepend the updates with the AS number received from the EBGP peer if you use the no prepend command. If you do not select no prepend, the default, the Local-AS adds to the first AS segment in the AS-PATH. If you use an inbound route-map to prepend the AS-PATH to the update from the peer, the Local-AS adds first.

Local-AS prepends before the route map to give the appearance that the update passed through a router in AS 200 before it reaches Router B.