Dynamic Interior gateway protocols (IGP) can be classified as two types:
■ Distance vector routing protocols
■ Link-state routing protocols
As we have seen, distance vector means that routes are advertised as vectors of distance and direction.
Distance is defined in terms of a metric such as hop count, and direction is simply the next hop router or exit interface. Distance vector protocols typically use the Bellman-Ford algorithm for the best-path route determination.
Some distance vector protocols periodically send complete routing tables to all connected neighbours. In large networks, these routing updates can become enormous, causing significant traffic on the links. Although the Bellman-Ford algorithm eventually accumulates enough knowledge to maintain a database of reachable networks, the algorithm does not allow a router to know the exact topology of an internetwork. The router only knows the routing information received from its neighbours.
Distance vector protocols use routers as signposts along the path to the final destination. The only information a router knows about a remote network is the distance or metric to reach that network and which path or interface to use to get there. Distance vector routing protocols do not have an actual map of the network topology. Distance vector protocols work best in situations where;
■ The network is simple and flat and does not require a hierarchical design.
■ The administrators do not have enough knowledge to configure and troubleshoot link-state protocols.
■ Specific types of networks, such as hub-and-spoke networks, are being implemented.
■ Worst-case convergence times in a network are not a concern.