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.

   (2017-12-02)