Route poisoning is a way to prevent
routing loops. Distance-vector routing protocols in computer networks use route poisoning to indicate to other routers that a route is no longer reachable and should be removed from their routing tables. A variation of route poisoning is " split horizonwith poison reverse" whereby a router sends updates with unreachable hop counts back to the sender for every route received to help prevent routing loops.
Some Distance-vector routing protocols such as RIP use a maximum hop count to determine how many routers traffic must go through to reach the destination. Each route has a hop count number assigned to it which is incremented as the routing information is passed from router to router. A route is considered unreachable if the hop count exceeds the maximum allowed. Route poisoning is a method of quickly removing outdated routing information from other router's routing tables by changing its hop count to be unreachable (higher than the maximum number of hops allowed) and sending a routing update.
In the case of the Routing Information Protocol (RIP), the maximum hop count is 15 so to perform route poisoning on a route its hop count is changed to 16, deeming it unreachable (sometimes referred to as an "Infinite metric") and a routing update is sent.
When a router receives a route poisoning, it sends an update back to the router from which it received the route poisoning, this is called "poison reverse". This is to ensure that all routers on a segment have received the poisoned route information.
[http://www.tcpipguide.com/free/t_RIPSpecialFeaturesForResolvingRIPAlgorithmProblems-2.htm The TCP-IP Guide, RIP Special Features For Resolving RIP Algorithm Problems, by Charles M. Kozierok]
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