Users Guide

Table 9. Redistributed Route Rules
Command Settings BGP Local Routing
Information Base
MED Advertised to Peer
WITH route-map metric-
type internal
MED Advertised to Peer
WITHOUT route-map
metric-type internal
redistribute isis (IGP cost =
20)
MED: IGP cost 20 MED = 20 MED = 0
redistribute isis route-map set
metric 50
MED: IGP cost 50 MED: 50 MED: 50 MED: 50 MED: 50
redistribute isis metric 100 MED: IGP cost 100 MED: 100 MED: 100
Ignore Router-ID in Best-Path Calculation
You can avoid unnecessary BGP best-path transitions between external paths under certain conditions. The bgp bestpath
router-id ignore command reduces network disruption caused by routing and forwarding plane changes and allows for
faster convergence.
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 22. Before and After 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.
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Border Gateway Protocol (BGP)