Distance Vector Routing Protocols

Distance vector as the name suggests uses distance between remote networks to determine the best path to a remote network. The distance vector metric is typically the hop. It’s not a measure of distance as such, rather a count of the number of routers in between the router and the destination network. Since a router performs functions on packets that pass through it, and these processes take time, routers cause a certain amount of latency in the network. So it’s fair to assume that the more routers in between the source and the destination, the more latency you can expect. This is how a true distance vector protocol works.

Using hop count as your only metric however doesn’t take in to account the bandwidth of the paths in the routing table. If you are using a hop count only distance vector protocol such as RIP within your network, where all the interconnects are T1 lines then this won’t cause you too much of a problem.

However consider that your network has two routes to a remote network. One of those routes has a hop count of 2, but is a T1 link, and the other is an ISDN line with a hop count of 1.The ISDN route would be chosen, by the router, over the T1 route since it has a lower hop count, even though it’s the slower of the two routes.

Fortunately not all distance vector routing protocols use hop count as their only metric. Cisco’s propriety IGRP is considered distance vector even though it uses bandwidth and delay as it’s default metric.

Distance vector protocols all have one thing in common. That is that they all send out their complete routing table at periodic intervals. They do this in the form of a broadcast made to all directly connected routers. The problem associated with this method is the increased bandwidth usage even though the network topology may have changed.

There are two distance vector algorithms used by the distance routing protocols supported by Cisco routers. They are Bellman-Ford algorithm, which is used by RIP v1, RIP v2 and IGRP, and the Diffusing Update Algorithm (DUAL) used by EIGRP.

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