Poison Reverse is often used within distance-vector routing to solve the count-to-infinity problem. In order to give the router enough time to propagate the poisoned route and to ensure that no routing loops occur while propagation occurs, the routers implement a hold-down mechanism.

When a router advertises a poisoned route to its neighbors, its neighbors break the rule of split horizon and send back to the originator the same poisoned route, called a poison reverse. With route poisoning, when a router detects that one of its connected routes has failed, the router will poison the route by assigning an infinite metric to it and advertising it to neighbors. Split horizon states that if a neighboring router sends a route to a router, the receiving router will not propagate this route back to the advertising router on the same interface. The main disadvantage of poison reverse is that it can significantly increase the size of routing announcements in certain fairly common network topologies, but it allows for the improvement of the overall efficiency of the network in case of faults. The effect of such an announcement is to immediately remove most looping routes before they can propagate through the network. Split-horizon routing with poison reverse is a variant of split-horizon route advertising in which a router actively advertises routes as unreachable over the interface over which they were learned by setting the route metric to infinite (16 for RIP). With the split-horizon rule in place, this particular loop scenario cannot happen, improving convergence time in complex, highly-redundant environments.

A would send the packet right back to B, creating a loop. However, if the link between B and C goes down, and B had received a route from A to C, B could end up using that route via A. On the surface, this seems redundant since B will never route via node A because the route costs more than the direct route from B to C. The links between the nodes are distinct point-to-point links.Īccording to the split-horizon rule, node A does not advertise its route for C (namely A to B to C) back to B. In this example, network node A routes packets to node B in order to reach node C. Whereas under split horizon N does not send any information through I, under poison reverse node N tells a white-lie.

Poison reverse rule: if a node N uses interface I to transmit to a given destination D, N sends through I the information that its cost-to-go to D is infinite.

Split horizon rule: if a node N uses interface I to transmit to a given destination D, N should not send through I new information about D.Route poisoning: if a node N learns that its route to a destination D is unreachable, inform that to all nodes in the network by sending them a message stating that the distance from N to D, as perceived by N, is infinite.The concept was suggested in 1974 by Torsten Cegrell, and originally implemented in the ARPANET-inspired Swedish network TIDAS. In computer networking, split-horizon route advertisement is a method of preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned. For the Domain Name System feature, see Split-horizon DNS.

Split horizon definition