Editing Distance-vector routing protocol (section)

==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===
[[Routing Information Protocol|RIP]] uses the [[Split horizon route advertisement|split horizon]] with 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 RFC|1058|section=2.2.2}}  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]].