Distance Vector Routing Protocols

Most routing protocols fall into one of two classes: distance vector or link state. The basics of distance vector routing protocols are examined here; the next section covers link state routing protocols. Distance vector algorithms are based on the work done of R. E. Bellman,¹ L. R. Ford, and D. R. Fulkerson² and for this reason occasionally are referred to as Bellman-Ford or Ford-Fulkerson algorithms.

The name distance vector is derived from the fact that routes are advertised as vectors of (distance, direction), where distance is defined in terms of a metric and direction is defined in terms of the next-hop router. For example, "Destination A is a distance of 5 hops away, in the direction of next-hop router X." As that statement implies, each router learns routes from its neighboring routers' perspectives and then advertises the routes from its own perspective. Because each router depends on its neighbors for information, which the neighbors in turn may have learned from their neighbors, and so on, distance vector routing is sometimes facetiously referred to as "routing by rumor."

Distance vector routing protocols include the following:

• Routing Information Protocol (RIP) for IP
• Xerox Networking System's XNS RIP
• Novell's IPX RIP
• Cisco's Internet Gateway Routing Protocol (IGRP)
• DEC's DNA Phase IV
• AppleTalk's Routing Table Maintenance Protocol (RTMP)