Dell Force10 Z9000 FTOS Configuration Guide for Z9000 System - Page 177

Figure 8-10. Dynamic changes when bgp asnotation command is disabled in the show running config 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 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 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 8-11 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. Border Gateway Protocol | 177