Poison Reverse and Triggered Updates

Poison reverseis when a router informs other routers that routes they were once capable of reaching via a particular interface are no longer reachable because the interface has gone down.

Routers normally react to a poison reverse message by immediately placing the poisoned routes into holddown instead of waiting for the invalid timer to expire. This saves convergence time, as much as 180 seconds (default invalid timer), depending on how soon after a regular update a poison reverse update arrives.

In Figure 2-10, RouterA's serial 1 interface is shut down. RouterA can no longer reach 168.71.9.0 and 168.71.8.0 out of serial 1.

Figure 2-10. Serial 1 on RouterA has been shut down.

By poisoning the routes ...

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