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Border Gateway Protocol IPv4 (BGPv4) | 221
Figure 10-10. Dynamic changes when bgp asnotation command is disabled in the show running
config
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 10-11 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.
AS NOTATION DISABLED
FTOS(conf-router_bgp)#no bgp asnotation
FTOS(conf-router_bgp)#sho conf
!
router bgp 100
bgp four-octet-as-support
neighbor 172.30.1.250 local-as 65057
<output truncated>
FTOS(conf-router_bgp)#do sho ip bgp
BGP table version is 28093, local router ID is 172.30.1.57
AS4 SUPPORT DISABLED
FTOS(conf-router_bgp)#no bgp four-octet-as-support
FTOS(conf-router_bgp)#sho conf
!
router bgp 100
neighbor 172.30.1.250 local-as 65057
FTOS(conf-router_bgp)#do show ip bgp
BGP table version is 28093, local router ID is 172.30.1.57