If you are large enough (say, you are a large national access or hosting provider), there is no single, even larger "The Internet, Inc." upstream provider you can use as your single link. You have to negotiate peering agreements with the other large actors so that you have reliable and affordable routes to all parts of the net. At this level you need things like BGP to know where to send traffic for a destination outside of your own net, because you could have several possible routes with different performance and/or price.
Unlike most LANs wich are either physically or logically (via the spanning tree protocol) structured like a tree, the core internet is a weighted graph.
Comments
If you are large enough (say, you are a large national access or hosting provider), there is no single, even larger "The Internet, Inc." upstream provider you can use as your single link. You have to negotiate peering agreements with the other large actors so that you have reliable and affordable routes to all parts of the net. At this level you need things like BGP to know where to send traffic for a destination outside of your own net, because you could have several possible routes with different performance and/or price.
Unlike most LANs wich are either physically or logically (via the spanning tree protocol) structured like a tree, the core internet is a weighted graph.