The Allow AS feature allows for routes to be received and processed even if the router detects its own ASN in the AS-Path. A router discards BGP network prefixes if it sees its ASN in the AS-Path as a loop prevention mechanism. Some network designs use a transit AS to provide connectivity to two different locations. BGP detects the network advertisements from the remote site as a loop and discards the route. The AS-Path loop check feature needs to be disabled to maintain connectivity in scenarios such as these.
On IOS nodes, the command neighbor ip-address allowas-in is placed under the address-family. IOS XR and NX-OS nodes use the BGP neighbor address-family configuration command allowas-in.