Administrator Guide
connection with Router C without immediately updating Router C’s conguration. Local-AS allows this behavior to happen by
allowing Router B to appear as if it still belongs to Router B’s old network (AS 200) as far as communicating with Router C is
concerned.
Figure 23. Before and After AS Number Migration with Local-AS Enabled
When you complete your migration, and you have recongured your network with the new information, disable this feature.
If you use the no prepend option, the Local-AS does not prepend to the updates received from the eBGP peer. If you do not
select
no prepend (the default), the Local-AS is added to the rst AS segment in the AS-PATH. If an inbound route-map is used
to prepend the as-path to the update from the peer, the Local-AS is added rst. For example, consider the topology described in the
previous illustration. If Router B has an inbound route-map applied on Router C to prepend "65001 65002" to the as-path, the
following events take place on Router B:
1. Receive and validate the update.
2. Prepend local-as 200 to as-path.
3. Prepend "65001 65002" to as-path.
Local-AS is prepended before the route-map to give an impression that update passed through a router in AS 200 before it reached
Router B.
162
Border Gateway Protocol IPv4 (BGPv4)