VMware

How to configure MPLS L3 with BGP AS OVERRIDE?

### BGP has a simple loop prevention mechanism for external BGP. When you see your own Autonomous system number in the AS path, we do not accept the prefix. This mechanism is fine for Internet routing but there are some other scenarios where this might be an issue. 

BGP has a simple loop prevention mechanism for external BGP. When you see your own Autonomous system number in the AS path, we do not accept the prefix. This mechanism is fine for Internet routing but there are some other scenarios where this might be an issue. 

[https://mpls.internetworks.in/2020/08/how-to-configure-mpls-l3-with-bgp-as.html](https://mpls.internetworks.in/2020/08/how-to-configure-mpls-l3-with-bgp-as.html)

In our topology, no routes get installed because the CE routers receive the routes with their own AS coming from other sites. (5500). So as per BGP, it will not install the routes in the BGP table. It’s simple if the customer has the same ASN at different sites, the CE routers drop the BGP routes. The CE routers drop the BGP update as it seems that it’s own ASN 5500 is in the update. This behavior is the default behavior of BGP and is a prevention mechanism against loops in BGP

https://preview.redd.it/1e1lodove2l51.png?width=400&format=png&auto=webp&s=07c23b2cf95a0a3b18b67a27647e92905db74226

However, we have an easier solution is available and it involves having the PE router replace the customer ASN in the AS path with the ASN of the service provider. The command that you need to configure on the PE router to override the [ASN is neighbor-address as-override.](https://mpls.internetworks.in/2020/08/how-to-configure-mpls-l3-with-bgp-as.html)


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