peer substitute-as (BGP multi-instance VPN instance IPv4 address family view)

Usage Scenario

In a L3VPN scenario, if the ASs to which two VPN sites belong use private AS numbers, the AS numbers of the two VPN sites may be the same. If a CE in a VPN site sends a VPN route to the connected PE using EBGP and the PE then sends the route to the remote PE, the remote CE will discard the route because the AS number carried by the route is the same as the local AS number. As a result, different sites of the same VPN cannot communicate. The peer substitute-as command can be used on the PE to enable AS number substitution to address this problem. After that, the PE replaces the AS number carried in the VPN route with the local AS number. As a result, the remote CE will not discard the route due to identical AS numbers.

On a BGP public network, two devices have the same AS number and the same EBGP peer. After one of the two devices learns a route of the other device from the EBGP peer, the route is discarded because it carries an AS number that is the same as the local one. To address this problem, run the peer substitute-as command on the EBGP peer.

Prerequisites

Enabling BGP AS number substitution may cause route loops in a CE multi-homing network. The peer soo command must be run to prevent a routing loop in a VPN site.

On BGP public networks, if three or more BGP peers form a ring network, the peer substitute-as command cannot be run; otherwise, a routing loop may occur.