[betterunix2]

BGP is the de facto routing protocol of the Internet. The basic problem that it solves is computing and distributing routes between independently operated networks. It is actually a pretty ramshackle protocol (among other things it usually uses TCP, which can create some interesting problems [1]) but it is the only widely deployed protocol that can handle the scale of the Internet.

In theory BGP can be used within a single network, though it is not well-suited to such an application except in very large networks. Facebook is known to use BGP internally [2]. Usually an "interior gateway protocol" like OSPF or IS-IS is better for internal routes, with BGP then distributing those routes to the external networks from the gateway routers (i.e. those routers that interconnect the networks). BGP can also be used for other, more niche applications within a single network, like setting up VPNs (VPLS, VPNv4, etc.).

[1] https://blog.benjojo.co.uk/post/bgp-stuck-routes-tcp-zero-wi...

[2] https://research.facebook.com/publications/running-bgp-in-da...