Flat Routing Protocols
Flat routing protocols distribute information as needed to any router that can be reached or receive information. No effort is made to organize the network or its traffic, only to discover the best route hop by hop to a destination by any path. Think of this as all routers sitting on a flat geometric plane. Routing Information Protocol (RIP) is an example of a flat routing protocol.
Hierarchichal Routing Protocols
Hierarchical routing protocols often group routers together by function into a hierarchy. A heirarchical routing protocol allows an administrator to make best use of his fast powerful routers in the backbone, and the slower, lower-powered routers may be used for network access at the edge of the network. The access routers form the first tier of the hierarchy, and the backbone routers form the second tier. Hierarchichal protocols make an effort to keep local traffic local, that is, they will not forward traffic to the backbone if it is not necessary to reach a destination. Some hierearchichal routing protocols also perform route aggregation to reduce the number of routes advertised (only summary routes are advertised).
Connections and data flow through the access routers, and only enter the backbone when trying to reach distant parts of the network that have no local connections other than the backbone routers. This allows traffic to flow freely locally, and concentrates long distance data onto the backbone links to flow efficiently to the opposite side minimizing congestion.
Open Shortest Path First (OSPF) and Intermediate-system to Intermediate-System (IS-IS) are two routing protocols that can be configured to organize a network hierarchically.