There are two main types of routing protocols:

■ Distance vector protocol: In a distance vector protocol, routing decisions are made on a hopby- hop basis. Each router relies on its neighbor routers to make the correct routing decisions. The router passes only the results of this decision (its routing table) to its neighbors. Distance vector protocols are typically slower to converge and do not scale well; however, they are easy to implement and maintain. Examples of distance vector protocols include RIPv1, RIPv2, and Interior Gateway Routing Protocol (IGRP).

■ Link-state protocol: Each router floods information about itself (its link states) either to all other routers in the network or to a part of the network (area). Each router makes its own routing decision based on all received information and using the shortest path first (SPF) algorithm (also called the Dijkstra algorithm), which calculates the shortest path to any destination. Link-state protocols are fast to converge, have less routing traffic overhead, and scale well. However, because of their complexity, link-state protocols are more difficult to implement and maintain. The IP link-state protocols are OSPF and Integrated IS-IS.

When a network is using a distance vector routing protocol, all the routers periodically send their routing tables, or a portion of their tables, to only their neighboring routers. In contrast, when a network is using a link-state routing protocol, each of the routers sends the state of its own interfaces (its links) to all other routers, or to all routers in a part of the network known as an area, only when there is a change. Routers running a hybrid protocol send changed information only when there is a change (similar to link-state protocols), but only to neighboring routers (similar to distance vector protocols).