A 911 PSAP needs a special extranet peering where routes legitimately return with the PSAP’s own ASN in the path, which normal loop detection would drop. How should allowas-in be treated?
Select an answer to reveal the explanation.
Short Explanation
Allowas-in is cutting a hole in the ‘don’t accept your own ASN’ safety net. Sometimes an extranet design needs that hole—but you cut it only on that one peer, not on every ISP uplink, or you’ve invited routing loops to the party.
Full Explanation
BGP drops updates that already contain the local ASN to prevent loops. Allowas-in relaxes that check for scenarios such as certain extranets or hub-and-spoke overlays where the ASN reappears by design. Because it weakens loop detection, it must be scoped to the specific peer and depth required—not enabled indiscriminately on public Internet edges. Weight and iBGP reflector tricks are not substitutes for a carefully limited allowas-in exception.