BGP Allowas-in feature Explained in 2022
BGP Allowas-in feature needs to be understood well in order to understand the BGP loop prevention behavior, But also, why the BGP Allowas-in configuration might create a dangerous situation, and what are the alternatives of BGP Allowas-in will be explained in this post.
What is the BGP Allowas-in feature?
BGP Allow-as-in feature is used to allow the BGP speaker to accept the BGP updates even if its own BGP AS number is in the AS-Path attribute.
By default EBGP loop prevention is, if any BGP speaker sees its own AS Number in the BGP update, then the update is rejected, thus the advertisement cannot be accepted. But there might be situations to accept the prefixes, thus there are two options to overcome this behavior.
Either accepting the BGP update even if the AS number is in the AS-Path list, with the BGP Allow AS feature or changing the behavior with the BGP AS Override feature.
Without BGP Allowas, let’s see what would happen.

In this topology, Customer BGP AS is AS 100. The customer has two locations.
Service Provider, in the middle, let’s say providing MPLS VPN service for the customer.
As you can understand from the topology, Service Provider Continue reading


