The peer substitute-as command enables a device to replace the AS number of the peer specified in the AS_Path attribute with the local AS number.
The undo peer substitute-as command disables AS number substitution.
By default, AS number substitution is disabled.
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.
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.
<HUAWEI> system-view [~HUAWEI] bgp 100 [*HUAWEI-bgp] peer 2001:DB8:1::1 as-number 200 [*HUAWEI-bgp] ipv4-family unicast [*HUAWEI-bgp-af-ipv4] peer 2001:DB8:1::1 enable [*HUAWEI-bgp-af-ipv4] peer 2001:DB8:1::1 substitute-as