Flat routing protocols propagate all routing information throughout the network, whereas hierarchical routing protocols divide large networks into smaller areas.

Flat Routing Protocols

Flat routing protocols have no means of limiting route propagation in a major network (within a Class A, B, or C network) environment. These protocols are typically classful distance vector protocols.

Hierarchical Routing Protocols

To solve the problems associated with flat routing protocols, additional features are implemented in hierarchical routing protocols to support large networks—for example, some support an areabased design. Hierarchical routing protocols are typically classless link-state protocols. classless means that routing updates include subnet masks in their routing updates; therefore, the routing protocol supports VLSM.