BGP allowas-in

Background Information

1. Sometimes, we need to override the BGP loop prevention mechanism and allow prefixes that contain its own AS.
2. A use case for this would be if you had two sites that used the same AS, but they are separated by an ISP, and both sites peer with the ISP.  The ISP is basically relaying the updates between the two sites.  This wouldn't work because each site will tag updates with its own AS, and then the remote site will drop it because it has the same AS.

Summary

1. Use allows-in neighbor policy to not drop prefixes that have its own AS.
2. Be aware of what effect this could have depending on the topology and policies already in place such as aggregation with as-set.  Check out as-override as an alternative.

router bgp 1

  address-family ipv4

    neigh 10.1.1.1 allowas-in

http://ccieblog.co.uk/bgp/bgp-as-override-vs-allow-as-in