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Dell Configuration Guide for the S4048–ON System 9.14.2.6

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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 1. Before and After AS Number Migration with Local-AS Enabled
Before and after illustration of AS number migration with Local-AS enabled.

When you complete your migration, and you have reconfigured 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 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 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.

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