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.

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

We have a network of two routers. Both routers are running RIP. R2 has advertised the 10.0.0.0/24 network to R1. Now consider what happens when the network 10.0.0.0/24 fails:

1. R2 removes the route to 10.0.0.0/24 from its routing table.

2. R2 advertises the 10.0.0.0/24 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.