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BGP Fundamentals

Chapter Description

In This sample chapter from Troubleshooting BGP: A Practical Guide to Understanding and Troubleshooting BGP, the authors cover BGP Messages and Inter-Router Communication, Basic BGP Configuration for IOS, IOS XR, and NX-OS, IBGP Rules, EBGP Rules, and BGP Route Aggregation

Allow AS

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.

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