distancevector

Distance-vector routing is a family of routing algorithms used in computer networks in which each router maintains a routing table containing the distance to reach every destination and the next hop to use. Routers periodically advertise their distance vectors to directly connected neighbors. The distance to a destination is the sum of the costs along the path, making the Bellman-Ford algorithm a common foundation for these protocols.

Operation: Each router stores a vector of destination costs and next hops. When a router receives a neighbor’s distance vector, it computes the cost to each destination via that neighbor and updates its own table if a shorter path is found. Updates are typically sent at regular intervals, though some protocols use triggered updates on topology changes. The classic metric is hop count.

Limitations: Distance-vector protocols can converge slowly and are prone to routing loops due to the count-to-infinity problem. Techniques such as split horizon, poison reverse, and hold-down timers mitigate loops. The most well-known distance-vector protocol is the Routing Information Protocol (RIP), including later variants that support subnetting and multicast updates (e.g., RIP v2).

Comparison: Unlike link-state routing, where each router builds a full map of the network and floods it to all nodes, distance-vector protocols exchange only neighboring information and require less memory on individual routers. They scale poorly to large or highly dynamic networks and are more susceptible to slow convergence, but remain simple to configure in small networks and legacy environments.

History: Distance-vector ideas trace to the Bellman-Ford algorithm, developed in the 1950s and 1960s. In networking, distance-vector protocols gained prominence in the 1980s with RIP and related implementations and continue to be used in some educational contexts and legacy equipment.