Split Horizon poison reverse and triggered updates

Poison Reverse and Triggered Updates

Distance vector protocols can attack the counting-to-infinity problem by ensuring that every router learns that the route has failed, through every means possible, as quickly as possible.  The next two loop-prevention features do just that and are defined as follows:

* Triggered update: When a route fails, do not wait for the next periodic update.  Instead, send an immediate triggered update listing the poisoned route.

* Poison reverse: When learning of a failed route, suspend split-horizon rules for that route, and advertise a poisoned route.

Problem: Counting to Infinity in a Redundant Network

Split horizon prevents the counting-to-infinity problem from occurring between two routers.  However, with redundant paths in an internetwork, split horizon alone does not prevent the counting-to-infinity problem.