In the example shown in the diagram below the service providers network is in AS1 and the customer wants both of his CE routers at different sites to use AS 2. When a route is sent from CE 1 to CE 2, it contains an AS_PATH attribute containing AS 2. When CE 2 sees that the AS_PATH attribute contains its own AS number, it rejects the route.
One solution to this problem is to configure PE 2 to override the AS_PATH attribute that contains AS 2. When this is enabled, the PE router determines when the AS_PATH attribute in a route intended for a neighbor CE contains the same AS number as the CE. When this is determined, the PE router substitutes its own AS number for the CE‘s in the AS_PATH attribute. The CE is then able to receive the route. The following addition conditions apply when this feature is in effect:
The following example describes the configuration of PE 2 required to enable Autonomous System number override for the BGP neighbor CE 2.
device(config)# router bgp
device(config-bgp)# local-as 1
device(config-bgp)# neighbor 10.2.2.1 remote-as 1
device(config-bgp)# neighbor 10.2.2.1 update-source loopback0
device(config-bgp)# address-family ipv4 unicast vrf VPN1
device(config-bgp-ipv4u-vrf)# neighbor 10.33.36.2 remote-as 2
device(config-bgp-ipv4u-vrf)# neighbor 10.33.36.2 as-override