Distance-vector routing protocol: Difference between revisions

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A '''distance-vector routing protocol''' in [[data networks]] determines the best route for data packets based on distance. Distance-vector routing protocols measure the distance by the number of [[Router (computing)|routers]] a packet has to pass, one router counts as one hop. Some distance-vector protocols also take into account [[network latency]] and other factors that influence traffic on a given route. To determine the best route across a network, routers, on which a distance-vector protocol is implemented, exchange information with one another, usually [[routing tables]] plus hop counts for destination networks and possibly other traffic information. Distance-vector routing protocols also require that a router informs its neighbours of [[network topology]] changes periodically.

A '''distance-vector routing protocol''' in [[data networks]] determines the best route for data packets based on distance. Distance-vector routing protocols measure the distance by the number of [[Router (computing)|routers]] a packet has to pass, one router counts as one hop. Some distance-vector protocols also take into account [[network latency]] and other factors that influence traffic on a given route. To determine the best route across a network, routers, on which a distance-vector protocol is implemented, exchange information with one another, usually [[routing tables]] plus hop counts for destination networks and possibly other traffic information. Distance-vector routing protocols also require that a router informs its neighbours of [[network topology]] changes periodically.

Distance-vector routing protocols use the [[Bellman–Ford algorithm]] and [[Ford–Fulkerson algorithm]] to calculate the best route. Another way of calculating the best route across a network is based on link cost, and is implemented through [[link-state routing protocol]]s.

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Routers that use distance-vector protocol determine the distance between themselves and a destination. The best route for [[Internet Protocol]] [[Network packet|packets]] that carry [[data]] across a [[data network]] is measured in terms of the numbers of [[Router (computing)|routers]] (hops) a packet has to pass to reach its destination network. Additionally some distance-vector protocols take into account other traffic information, such as [[network latency]]. To establish the best route, routers regularly exchange information with neighbouring routers, usually their [[routing table]], hop count for a destination network and possibly other traffic related information. Routers that implement distance-vector protocol rely purely on the information provided to them by other routers, and do not assess the [[network topology]].<ref>{{Cite book|title= Network+ Guide to Networks|author =Tamara Dean |publisher= Cengage Learning|year=2009 |isbn= 9781423902454|pages=274}}</ref>

Distance-vector protocols update the routing tables of routers and determine the route on which a packet will be sent by the ''next hop'' which is the exit interface of the router and the IP address of the interface of the receiving router. Distance is a measure of the cost to reach a certain node. The least cost route between any two nodes is the route with minimum distance.

Updates are performed periodically in a distance-vector protocol where all or part of a router's routing table is sent to all its neighbours that are configured to use the same distance-vector routing protocol. Once a router has this information it is able to amend its own routing table to reflect the changes and then inform its neighbours of the changes. This process has been described as ‘routing by rumour’ because routers are relying on the information they receive from other routers and cannot determine if the information is actually valid and true. There are a number of features which can be used to help with instability and inaccurate routing information.

== Development of distance-vector routing ==

The oldest [[routing protocol]], and the oldest distance-vector protocol, is version 1 of the [[Routing Information Protocol]] (RIPv1). RIPv1 was formally standardised in 1988.<ref>{{Cite web|url=https://tools.ietf.org/html/rfc1058.html|title=Routing Information Protocol|last=Hedrick|first=C. L.|date=|website=tools.ietf.org|language=en|rfc=1058|archive-url=|archive-date=|dead-url=|access-date=2019-04-16}}</ref> It establishes the shortest path across a network purely on the basis of the hops, that is numbers of routers that need to be passed to reach the destination network. RIP is a [[interior gateway protocol]], so it can be used in [[local area networks]] (LANs) on interior or border routers. Routers with RIPv1 implementation exchange their routing tables with neighbouring routers by [[Broadcasting (networking)|broadcasting]] a RIPv1 packet every 30 second into all connected networks. RIPv1 is not suitable for large networks as it limits the number of hops to 15. This hop limit was introduced to avoid routing loops, but also means that networks that are connected through more than 15 routers are unreachable.<ref>{{Cite book|title= Network+ Guide to Networks|author =Tamara Dean |publisher= Cengage Learning|year=2009 |isbn= 9781423902454|pages=274}}</ref>

The distance-vector protocol designed for use in [[wide area network]]s (WANs) is the [[Border Gateway Protocol]] (BGP). BGP is a [[exterior gateway protocol]] and therefore implemented on border and exterior routers on the [[Internet]]. It exchanges information between routers through a [[Transmission Control Protocol]] (TCP) session. Routers with BGP implementation determine the shortest path across a network based on a range of factors other than hops. BGP can also be configured by administrators so that certain routes are preferred or avoided. BGP is used by [[internet service providers]] (ISPs) and telecommunication companies.<ref>{{Cite book|title= Network+ Guide to Networks|author =Tamara Dean |publisher= Cengage Learning|year=2009 |isbn= 9781423902454|pages=274-275274–275}}</ref>

Among the distance-vector protocols that have been described as a hybrid, because it uses routing methods associated with [[link-state routing protocol]]s, is the proprietary [[Enhanced Interior Gateway Routing Protocol]] (EIGRP). It was developed by [[Cisco]] in the 1980s and was designed to offer better convergence and cause less network traffic between routers than the link-state routing protocol [[Open Shortest Path First]] (OSPF).<ref>{{Cite book|title= Network+ Guide to Networks|author =Tamara Dean |publisher= Cengage Learning|year=2009 |isbn= 9781423902454|pages=275}}</ref>

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===Workarounds and solutions===

[[Routing Information Protocol|RIP]] uses the [[Split horizon route advertisement|split horizon]] with [[Split horizon route advertisement |poison reverse]] technique to reduce the chance of forming loops and uses a maximum number of hops to counter the 'count-to-infinity' problem. These measures avoid the formation of routing loops in some, but not all, cases.<ref>{{IETF RFC|1058}}, Section 2.2.2</ref> The addition of a ''hold time'' (refusing route updates for a few minutes after a route retraction) avoids loop formation in virtually all cases, but causes a significant increase in convergence times.

More recently, a number of loop-free distance vector protocols have been developed — notable examples are [[EIGRP]], [[DSDV]] and [[Babel (protocol)|Babel]]. These avoid loop formation in all cases, but suffer from increased complexity, and their deployment has been slowed down by the success of [[link-state routing protocol]]s such as [[OSPF]].