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Route poisoning is a method that prevents a certain network from sending data packets to a path destination that has already become invalid. This is done when a distance vector routing protocol sees an invalid route or one with large routing loops. A route is considered unreachable if it exceeds the maximum allowed. So the routing protocol simply informs all the routers connected in the network that a certain path is invalid by saying that it has a hop count that exceeds the maximum allowable.
For example, in Routing Information Protocol (RIP) the maximum is 15, so the hop count of the route to be poisoned is set to 16, making this path unreachable and will no longer be used for routing data.
Route poisoning works as follows: When a certain network path has been found to be unreachable by a certain router, the router will set the hop count of that path to 1 more than the maximum allowable count and then sends this to all the nodes in the network, telling them to remove the poisoned route as option in their routing tables.
Route poisoning is a good way to avoid inconsistent updates, and when used in conjunction with triggered updates will actually help to speed up network convergence because routers no longer have to wait before they can advertise that a certain route has been poisoned.