FTOS Configuration Guide for the C-Series - Force10 Networks

AS Number Migration
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 need to be updated to maintain network reachability. With this feature you can transparently
change the AS number of entire BGP network and ensure that the routes are propagated throughout the
network while the migration is in progress. 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.
Figure 343 shows a scenario where Router A, Router B and Router C belong to AS 100, 200, 300
respectively. Router A acquired Router B; Router B has Router C as its customer. When Router B is
migrating to Router A, it needs to maintain the connection with Router C without immediately updating
Router C's configuration. Local-AS allows this 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 343 Local-AS Scenario
AS 100
AS 200
Router A
Router B
Before Migration
AS 100
AS 100
Router A
Router B
Local AS
200
Router C
Router C
472 Border Gateway Protocol (BGP)
AS 300
AS 300
After Migration, with Local-AS enabled