192 | Border Gateway Protocol
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Figure 9-11. Local-AS Scenario
When you complete your migration, and you have reconfigured your network with the new information
you must disable this feature.
If the “no prepend” option is used, the local-as will not be prepended to the updates received from the
eBGP peer. If “no prepend” is not selected (the default), the local-as is added to the first 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 first. For example, consider the topology described in Figure 9-11. If Router B has an inbound
route-map applied on Router C to prepend "65001 65002" to the as-path, the following events will 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 thru a router in AS
200 before it reached Router B.
Router A
Router B
Router C
AS 200
AS 300
AS 100
Router C
AS 300
AS 100
Router B
Local AS
200
AS 100
Router A
Before Migration
fter Migration, with Local-AS enabled
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