AS Number Migration
With this feature you can transparently change the AS number of an entire BGP network and ensure that
the routes are propagated throughout the network while the migration is in progress.
When migrating one AS to another, perhaps combining ASs, an eBGP network may lose its routing to an
iBGP if the ASN changes. Migration can be difficult as all the iBGP and eBGP peers of the migrating
network must be updated to maintain network reachability. Essentially, Local-AS provides a capability to
the BGP speaker to operate as if it belongs to "virtual" AS network besides its physical AS network.
The following illustration shows a scenario where 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 customer. 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 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
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Border Gateway Protocol IPv4 (BGPv4)
