Connectivity 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
conguration. 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.
Congure Border Gateway Protocol
BGP is disabled by default. To enable the BGP process and start to exchange information, assign an AS number and use commands in
ROUTER-BGP mode to congure a BGP neighbor.
BGP neighbor
adjacency changes
All BGP neighbor changes are logged
Fast external fallover Enabled
Graceful restart Disabled
Local preference 100
4-byte AS Enabled
Layer 3 437