DELL S50V User Manual

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

FTOS(conf-router_bgp)#do sho ip bgp

BGP table version is 28093,

local router ID is 172.30.1.57

AS4 SUPPORT DISABLED