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Route poisoning

Route poisoning definition

Route poisoning is a computer networking technique to prevent routing loops in dynamic routing protocols (such as the Routing Information Protocol or RIP) and maintain network stability.

See also: broadcast storm, spanning tree protocol, switching loop, network congestion

How route poisoning works

A routing loop is a situation where data packets circulate endlessly through routers in a network without reaching their destination. Loops can lead to increased traffic and congestion — in extreme cases, an unresolved loop may even lead to a broadcast storm.

Typically, when a router determines that a route is no longer valid, it informs the neighboring routers of the failure. In route poisoning, however, the router goes a step further — it advertises the route with an infinite metric (a set value that denotes an unusable route).

For example, the default infinite metric for the Routing Information Protocol (RIP) is 16. When a router using this protocol determines that a route has failed, it sends an update with a metric of 16 for that route, indicating that the route is unreachable.

After announcing a route as unreachable, the router typically waits for a specific hold-down timer before considering alternative paths or accepting updates for the failed route. This helps prevent transient issues from causing unnecessary route changes and consuming network resources.