Poisoned reverse

Richard, The is no single mechanism is adequate for routing loop avoidance. And route poisoning alone doesn't cut it, and I thinink you already know this. Along with triggered updates, route poisoning accomplishes good convergence time. Regarding poison reverse, The poison reverse is an acknoledgment of reception of route poisoning. And ALL routers on the segment connected to the router issuing route poison MUST send back a poison reverse. If the routers dont send poison reverse, this means that the router has lost the right way to reach the dead network. This is considered neccessary in distance vector protocols because they don't see the full scope of the network.

I hope this helps.

Mohammed

Very interesting question, I think its quite confusing myself, but I think the answer is that the poison reverse is a required rule and the neighbour router has no concept of a router being turned off. Its a very simple algorithm. So its just sends it anyway.

The point of poison reverse is to inform the dead router that there is no route via the neighbour to the dead routers network.

Hi,

Poison Reverse prevents loose loops.

Think of something like this |- rtrA rtrB rtrC -| 172.16.0.0/16

if rtrC link to net 172.16.0.0 fails it sends a flash update with a poisoned route to rtrB, which in turns sends a poisoned route to either rtrC and rtrA which also responds to rtrB with a poisoned route. The poisoned route is then advertised with the scheduled updates for xx times.

Poison Reverse can also work ( cisco routers don't do this if I remember correctly ) with Split Horizon advertising incoming routes out the SAME interface BUT with an infinite metric.

Regards, Gabriele