BGP - eBGP loop prevention mechanism same AS number

BGP has a simple loop prevention mechanism that is employed for eBGP. If an eBGP router sees its own AS number in the AS path advertised from its neighboring eBGP routers, the prefix is not accepted.

This feature can be overridden by using either the BGP AS Override or the BGP Allow-AS-In feature.

Links

https://networklessons.com/bgp/mpls-layer-3-vpn-bgp-override/