BGP AS Override needs to be understood well in order to understand the BGP loop prevention behavior, But why BGP AS Override might create a dangerous situation, and what are the alternatives of BGP AS Override will be explained in this post.
What is BGP AS Override
BGP AS Override feature is used to change the AS number or numbers in the AS Path attribute. Without BGP AS-Override, 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 is running EBGP with the Customer, because they have different BGP Autonomous Systems.
The service provider in the above topology has BGP AS 200.
Left customer router, when it advertises BGP update message to the R2, R2 sends to R3 and when R3 sends to R4, R4 wouldn’t accept the BGP update,
When R4 receives that update, it will check the AS-Path attribute and would see its own BGP AS number in the AS Path.
Thus is by default rejected, due to EBGP loop prevention.
If the router sees its Continue reading