BGP that stands for border gateway protocol came into existence because of the limitations faced by EGP which stands for the exterior gateway protocols. EGP was a type of protocol used for inter autonomous system routing but now, it is the TYPE of exterior gateway protocols. EGPs are ISP level routing protocols, not strictly ISP but saying this is just to give the idea of how huge it is. Other huge companies also do use this routing protocol. As defined in the RFC 4271, BGP is an Inter-Autonomous meant to exchange reachability information with other systems.An autonomous system could be a set of routers under a single administration.
These single ASes could use different interior gateway protocols (IGP) for communication within it's AS like OSPF, EIGRP, RIP and static routes. But for routing information to be exchanged across different autonomous systems, BGP is THE routing protocol.The BGP comes up with a reachability graph provided by it's speaking system that includes information of the list of all autonomous systems reached.The figure below is a trivial view of the internet with different Autonomous Systems at different locations in the world. These ASes make routing decisions based on BGP and it's metrics.
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