Users Guide

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
conguration. 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 rst 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 rst.
If Router B has an inbound route-map applied on Router C to prepend 65001 65002 to the AS-PATH, these events take place on Router B:
• Receive and validate the update.
• Prepend local-as 200 to AS-PATH.
• Prepend 65001 65002 to AS-PATH.
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.
Graceful restart
OS10 oers graceful restart capability for BGP in helper mode only.
A BGP router whose neighbor is restarting is called a "helper."
If graceful restart is enabled on the restarting router, during restart, the helper maintains the routes that it has learnt from its neighbor.
After the switch over, the graceful restart operation begins. Both routers reestablish their neighbor relationship and exchange their BGP
routes again. The helper continues to forward prexes pointing to the restarting peer, and the restarting router continues to forward trac
to its peers even though those neighbor relationships are restarting. When the restarting router receives all route updates from all BGP
peers that are graceful restart capable, the graceful restart is complete. BGP sessions become operational again.
504
Layer 3