Routing protocols keep track of changes on the network and share these changes with routers throughout the network so it is very important choosing the correct routing protocols.
Routing protocols are categorized into two major types: distance vector and link state.
Distance vector- routing protocols are designed to run on small networks (usually fewer than 100 routers). These are generally easier to configure and require less maintenance than link state protocols.
Distance vector routing protocols use a hop count to determine the best path through an internetwork. The hop count is simply a measure of the number of routers a packet must cross to get from host A to host B. Distance vector protocols always choose the route with the fewest number of hops as the best route. This can be a problem when the best route to a destination is not the route with the least number of hops. Convergence time is the amount of time it takes to propagate changes in network topology throughout the internetwork. In a large environment with many routers, this can greatly affect CPU and bandwidth utilization. They also take longer to converge than do link state protocols.
Distance vector routing protocols are great for a small environment, but when it comes to enterprise networking, you must deploy link state protocols. Examples of distance vector routing protocols include RIP and IGRP.
Link state- routing protocols are designed to operate in large, enterprise-level networks. Link state routing protocols are very complex and are much more difficult to configure, maintain, and troubleshoot than distance vector routing protocols. However, link state routing protocols overcome many of the shortcomings of distance vector protocols.
Link state protocols use a different algorithm than distance vector protocols for calculating the best path to a destination. This algorithm takes into account bandwidth as well as other factors when calculating the best path for a packet to traverse the network. Additionally, link state convergence occurs faster than distance vector convergence. This is because link state establishes a neighbor relationship with directly connected peers and shares routing information with its neighbors only when there are changes in the network topology.
Link state protocols exchange periodically (about every 90 seconds) send their updates to every router on the network. Also, link state routing protocols only send updates to neighboring routers, unlike distance vector protocols, which send the entire routing table. Examples of link state protocols include OSPF and NLSP.