Route poisoning is a method to prevent a router from sending packets through a route that has become invalid within

computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...

Distance-vector routing protocol A distance-vector routing protocol in data networks determines the best route for data packets based on distance. Distance-vector routing protocols measure the distance by the number of routers a packet has to pass; one router counts as one hop ...

s in computer networks use route poisoning to indicate to other routers that a route is no longer reachable and should not be considered from their routing tables. Unlike the '' split horizon with poison reverse'', route poisoning provides for sending updates with unreachable hop counts immediately to all the nodes in the network. When the protocol detects an invalid route, all of the routers in the network are informed that the bad route has an infinite (∞) route metric. This makes all nodes on the invalid route seem infinitely distant, preventing any of the routers from sending packets over the invalid route. Some distance-vector routing protocols, such as RIP, use a maximum hop count to determine how many routers the 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 Route poisoning is a method of quickly forgetting outdated routing information from other router's routing tables by changing its hop count to be unreachable (higher than the maximum number of hops allowed) and sending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 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, and a routing update is sent. If these updates are lost, some nodes in the network would not be informed that a route is invalid, so they could attempt to send packets over the bad route and cause a problem known as a

routing loop A routing loop is a common problem with various types of networks, particularly computer networks. They are formed when an error occurs in the operation of the routing algorithm, and as a result, in a group of nodes, the path to a particular destin ...

. Therefore route poisoning is used in conjunction with

holddown Holddown works by having each router start a timer when they first receive information about a network that is unreachable. Until the timer expires, the router will discard any subsequent route messages that indicate the route is in fact reachable ...

s to keep update messages from falsely reinstating the validity of a bad route. This prevents routing loops, improving the overall efficiency of the network.

References

The TCP-IP Guide, RIP Special Features For Resolving RIP Algorithm Problems, by Charles M. Kozierok

RFC 1058: Routing Information Protocol, by C. Hedrick, Rutgers University (June 1988)
{{reflist Internet Standards Internet protocols Routing protocols