Route Poisoning

When a route fails, distance vector routing protocols risk causing routing loops until every router in the internetwork believes and knows that the original route has failed.

Distance vector protocols spread the bad news about a route failure by poisoning the route.  Route poisoning refers to the practice of advertising a route, but with a special metric value called infinity.  Routers consider routes advertised with an infinite metric to have failed.  Each distance vector routing protocol uses the concept of an actual metric value that represents infinity.  RIP defines infinity as 16.

Problem: Counting to Infinity over a Single Link

Distance vector routing protocols risk causing routing loops during the time between when the first router realizes a route has failed until all the routers know that the route has failed.  Without the loop-prevention mechanisms, distance vector protocols can experience a problem called counting to infinity.

Counting to infinity causes two related problems:

* Packets may loop around the internetwork while the routers count to infinity, with the bandwidth consumed by the looping packets crippling an internetwork.

* The counting-to-infinity process may take several minutes, meaning that the looping could cause users to believe that the network has failed.

When routers count to infinity, they collectively keep changing their minds about the metric of a failed route.  The metric grows until it reaches infinity, at which point the routers finally believe that the route has failed.