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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.

Introduction
This document describes a situation where two branch routers connect via an ISP and run BGP between them.

Prerequisites
Requirements
Cisco recommends that you have knowledge of these topics:

Internet Service Provider (ISP)
Border Gateway Protocol (BGP)
Components Used
This document is not restricted to specific software and hardware versions.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

Conventions
Refer to Cisco Technical Tips Conventions for more information on document conventions.

Background Information
This document describes a scenario where two branch routers are connected via an Internet Server Provider (ISP) and run Border Gateway Protocol (BGP) between them. The two branch routers (R1 and R2), though at different locations, share the same AS number. Once the routes arrive from a branch (R1 in this case) to the Service Provider (SP) network, they can be tagged with the user AS. Once the SP passes it to the other branch router (R2), by default, the routes can be dropped if the other branch also runs BGP with the SP and uses the same AS number. In this scenario, the neighbor allowas-in command is issued in order to allow BGP at the other side to inject updates. This document provides a sample configuration that helps you to understand the Allowas-in feature in BGP.

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.

Allowas In :
This feature allows for routes to be received and processed even if router detects its own ASN in AS-Path. A router discards BGP network prefixes if it sees its ASN in AS-Path as a loop prevention mechanism.