Distance-vector routing protocol: Difference between revisions

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{{short description|Class of routing protocols}}

{{Multiple issues|

{{More footnotes needed|date=September 2010}}

{{technical|date=November 2013}}

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* [[Routing Information Protocol#RIPng|Routing Information Protocol Next Generation]] (RIPng), an extension of RIP version 2 with support for [[IPv6]]

* [[Interior Gateway Routing Protocol]] (IGRP)

* [[Enhanced Interior Gateway Routing Protocol]] (EIGRP)

==Methodology==

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]] acrossthrough a [[data network]] is measured in terms of the numbers of [[Router (computing)|routers]] (hops) a packet has to pass through 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|url= https://archive.org/details/networkguidetone00dean_142|url-access= limited|author =Tamara Dean |publisher= Cengage Learning|year=2009 |isbn= 9781423902454|pages=[https://archive.org/details/networkguidetone00dean_142/page/n294 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.

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==Count to infinity problem==

The [[Bellman–Ford algorithm]] does not prevent [[routing loop]]s from happening and suffers from the '''count to infinity problem'''. The core of the count- to- infinity problem is that if A tells B that it has a path somewhere, there is no way for B to know if the path has B as a part of it. To see the problem, imagine a subnet connected like A–B–C–D–E–F, and let the metric between the routers be "number of jumps". Now suppose that A is taken offline. In the vector-update-process B notices that the route to A, which was distance 1, is down – B does not receive the vector update from A. The problem is, B also gets an update from C, and C is still not aware of the fact that A is down – so it tells B that A is only two jumps from C (C to B to A). Since B doesn't know that the path from C to A is through itself (B), it updates its table with the new value "B to A = 2 + 1". Later on, B forwards the update to C and due to the fact that A is reachable through B (From C's point of view), C decides to update its table to "C to A = 3 + 1". This slowly propagates through the network until it becomes infinity (in which case the algorithm corrects itself, due to the relaxation property of Bellman-Ford).

===Workarounds and solutions===

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|colspan=5| At this point, all the routers (A, B, C, D) have new "shortest-paths" for their DV (the list of distances that are from them to another router via a neighbor). They each broadcast this new DV to all their neighbors: A to B and C, B to C and A, C to A, B, and D, and D to C. As each of these neighbors receives this information, they now recalculate the shortest path using it.

For example: A receives a DV from C that tells A there is a path via C to D, with a distance (or cost) of 5. Since the current "shortest-path" to C is 23, then A knows it has a path to D that costs 23+5=28. As there are no other shorter paths that A knows about, it puts this as its current estimate for the shortest-path from itself (A) to D, via C.

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==Further reading==

* [http://docwiki.cisco.com/wiki/Routing_Basics#Link-State_Versus_Distance_Vector Section "Link-State Versus Distance Vector"] {{Webarchive|url=https://web.archive.org/web/20101214043441/http://docwiki.cisco.com/wiki/Routing_Basics#Link-State_Versus_Distance_Vector |date=2010-12-14 }} in the Chapter "Routing Basics" in the [[Cisco Systems|Cisco]] "Internetworking Technology Handbook"

* [httphttps://authorscsc-knu.phptrgithub.comio/tanenbaumcn4sys-prog/books/Andrew%20S.%20Tanenbaum%20-%20Computer%20Networks.pdf Section 5.2 "Routing Algorithms"] in Chapter "5 THE NETWORK LAYER" from "Computer Networks" 4th.5th Edition by Andrew S. Tanenbaum and David J. Wetherall

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