Other meanings of Distance-vector routing protocol
Computer Networking
A distance-vector routing protocol is a class of computer network routing protocols in which each router maintains a table of distances (metrics) to every destination and periodically shares this table with its directly connected neighbors. The protocol operates on the principle that each router learns routes indirectly: it trusts its neighbors' advertised distances and adds the cost of the link to that neighbor to compute its own distance. This iterative exchange of information converges to the shortest paths, but it is also susceptible to routing loops and slow convergence, which have led to enhancements such as split horizon and route poisoning.
The distance-vector algorithm is based on the Bellman-Ford equation, where each router computes the shortest path to a destination by taking the minimum over all neighbors of the sum of the neighbor's advertised distance and the link cost to that neighbor1. Each router maintains a routing table with entries for each destination, including the next hop and the total distance. Periodically, every router sends its entire routing table to its neighbors, which then update their own tables if a better route is found. This process is repeated until no further changes occur, at which point the network is said to have converged.
The metric used can be hop count, delay, or a composite cost, depending on the protocol. For example, RIP uses hop count, while IGRP uses a composite of bandwidth and delay. The algorithm is distributed and asynchronous, meaning routers do not need a global view of the network.
Distance-vector protocols are prone to routing loops and slow convergence, especially when a link fails. The classic count-to-infinity problem occurs when routers incrementally increase the distance to an unreachable destination until the metric reaches a predefined maximum (infinity), which can take many iterations2. To mitigate this, protocols employ techniques such as split horizon, which prevents a router from advertising a route back to the neighbor from which it learned it, and route poisoning, which explicitly marks a route as unreachable by advertising an infinite metric.
Hold-down timers are also used to prevent routers from accepting new routes for a period after a failure, reducing the risk of loops. Despite these measures, convergence time in distance-vector protocols is generally slower than in link-state protocols, making them less suitable for large, dynamic networks.
The most widely deployed distance-vector protocol is the Routing Information Protocol (RIP), which is defined in RFC 1058 and uses hop count as its metric, with a maximum of 15 hops (16 is considered infinity)3. RIP is simple to configure and was historically used in small networks. Its successor, RIPng, extends RIP to support IPv6. Another notable protocol is Interior Gateway Routing Protocol (IGRP), a Cisco proprietary protocol that uses a composite metric and supports multiple paths. The Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector protocol that incorporates features of link-state protocols, such as maintaining neighbor adjacencies and using a diffusing update algorithm (DUAL) to guarantee loop-free paths at every instant.
In wireless mesh networks, protocols like Babel and the Ad hoc On-Demand Distance Vector (AODV) protocol adapt the distance-vector concept to dynamic topologies, often using reactive route discovery.
Early distance-vector routing was used in the ARPANET in 1969, but it was soon replaced by link-state routing due to its slow convergence and bandwidth consumption4. The term "distance-vector" was coined by Ford and Fulkerson in their 1962 paper on the Bellman-Ford algorithm, which is the mathematical foundation of these protocols. A lesser-known variant is the path-vector protocol, used in BGP, which is an extension of distance-vector that carries the full path to a destination to avoid loops. Some distance-vector protocols, such as DSDV (Destination-Sequenced Distance-Vector), are designed for mobile ad hoc networks and use sequence numbers to prevent stale routes. The count-to-infinity problem can be visualized with the "two-node loop" example, where two routers repeatedly advertise increasing distances to each other, a classic teaching tool in networking courses.
Distance-vector protocols are a foundational concept in computer networking, illustrating the trade-off between simplicity and convergence speed.
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