BGP

Border Gateway Protocol (BGP) is an inter-AS protocol, meaning that it can be deployed within and between autonomous systems (AS). An autonomous system is a set of routers under a single technical administration. An AS uses an interior gateway protocol and common metrics to route packets within an AS; it uses an exterior routing protocol to route packets to other ASs.

BGP sends update messages that consist of network number-AS path pairs. The AS path contains the string of ASs through which the specified network can be reached. An AS path has some structure in order to represent the results of aggregating dissimilar routes. These update messages are sent over TCP transport mechanism to ensure reliable delivery. BGP contrasts with IGPs, which build their own reliability on top of a datagram service.

As a path-vector routing protocol, BGP limits the distribution of router reachability information to its peer or neighbor routers.

You can run BGP over a route-based VPN by enabling BGP on a virtual tunnel interface (VTI).