Route poisoning explained

Another method employed by distance vector routing protocols to prevent routing loops is route poisoning. When a router detects that one of its directly connected routes has failed, it will advertise a failed route with an infinite metric (“poisoning the route”). Routers who receive the routing update will consider the route as failed and remove it from their routing tables.

Each routing protocol has its own definition of an infinite metric. In the case of RIP the infinite metric is 16.


To better understand how route poisoning works, consider the following example:

how route poisoning works

We have a network of two routers. Both routers are running RIP. R2 has advertised the network to R1. Now consider what happens when the network fails:

1. R2 removes the route to from its routing table.

2. R2 advertises the network with an infinitive metric (16) to R1 (“route poisoning”).

3. R1 receives the update and knows that the network has failed. It will remove the route from its routing table.

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