3.1 Routing
3.1.4 Distance-vector routing protocols
Routing protocols may be classified as either distance-vector or link-state routing protocols. These classifications describe the algorithm, or formula, that routers use to calculate and exchange routing information. Distance-vector routing protocols are based on the Bellman-Ford algorithm (also known as a distance-vector algorithm).

Routers configured to use a distance-vector routing protocol typically send their complete routing table at regular intervals to neighbor routers. In fact, simple distance-vector protocols, such as RIP and IGRP, broadcast (or multicast) their routing table out all configured interfaces, by default. Routers that use these protocols do not actually identify their neighbors for direct communication.

A neighbor router receiving the broadcast update examines it and compares the information to its current routing table. Routes to new networks, or routes to known networks with better metrics, are inserted in the table. The neighbor then broadcasts its routing table, which includes any updated routes.

Distance-vector routing protocols are concerned with the distance and vector (direction) of destination networks. Before sending an update, each router adds its own distance value to the route's metric. When a router receives an update, it maps the learned network to the receiving interface. The router then uses that interface to reach those destinations.

Simple distance-vector routing protocols enjoy two major benefits over link-state protocols. They are relatively easy to configure, and they generally use less memory and processing power. RIPv1 has the added advantage of almost universal support among all routing software and is often used as a common denominator in mixed-vendor or legacy routing environments.

Simple distance-vector routing protocols do not scale as well as their link-state counterparts. RIPv1 and IGRP are classful routing protocols (they do not send subnet information in updates), so they can not support scalability features such as Variable Length Subnet Masking (VLSM) or supernetting. In general, simple distance-vector routing protocols converge more slowly than link-state protocols. Because complex and scalable internetworks demand routing protocols that are quick to achieve convergence (when all routers agree on the state of the network's topology), distance-vector protocols often are not appropriate. Finally, RIP restricts networks from growing beyond 15 hops between any two destinations, a limitation that proves too stifling for today's large networks. IGRP overcomes this limitation by supporting a 255-hop maximum. However, IGRP is a Cisco-proprietary protocol and therefore cannot support a multi-vendor routing environment.

Because of the limitations of simple distance-vector routing protocols, network administrators often turn to link-state routing in complex internetworks.