hierarchical routing
A way of simplifying routing a large network like the Internet by breaking it into a hierarchy of smaller networks where each level is responsible for its own routing. The Internet has three levels: backbone networks, mid-level networks (or transit networks) and stub networks. The backbones know how to route between the mid-levels, the mid-levels know how to route between autonomous systems (sites) and each site knows how to route internally.
Routers at each level cooperate by exchanging routing information. Typically, between mid-level networks this is via Exterior Gateway Protocol and within sites via Interior Gateway Protocol.