BGP Allow-AS in

When implementing the BGP routing protocol, in some cases, the eBGP loop prevention mechanism may cause undesired behavior. Take a look at this MPLS VPN topology for instance:

In this scenario, eBGP is running between the CE and PE routers. Both CE routers are in the same AS but the AS is discontiguous. This means that any routes they share must be shared via eBGP through the MPLS core network. As a result, the CE routers will receive prefixes where AS 12 is included in the AS path.

Under normal circumstances, such prefixes wouldn't be accepted. However, this can be resolved by using the Allow-AS in feature. This can be done by issuing the allowas-in keyword on the neighbor command on the CE routers.

This results in the CE router accepting prefixes with their own AS number in the AS path.

This problem can also be resolved using the BGP AS Override feature.

Links:

https://networklessons.com/bgp/mpls-layer-3-vpn-bgp-allow-as-in