The Holddown Process and Holddown Timer

Holddown prevents the looping and counting-to-infinity problem.  Distance vector protocols use holddown to specifically prevent the loops created by the counting-to-infinity problems that occur in redundant internetworks.

As soon as the route is considered to be down, hold it down for a while to give the routers time to make sure every router knows that the route has failed.

The holddown process tells a router to ignore new information about the failed route, for a time period called the holddown time, as counted using the holddown timer.  The holddown process can be summarized as follows:

After hearing a poisoned route, start a holddown timer for that one route.  Until the timer expires, do not believe any other routing information about the failed route, because believing that information may cause a routing loop.  However, information learned from the neighbor that originally advertised the working route can be believed before the holddown timer expires.

Distance Vector Summary

* During periods of stability, routers send periodic full routing updates based on a short update timer (the RIP default is 30 seconds).  The updates list all known routes except the routes omitted because of split-horizon rules.

* When changes occur that cause a route to fail, the router that notices the failure reacts by immediately sending triggered partial updates, listing only the newly poisoned (failed) routes, with an infinite metric.

* Other routers that hear the poisoned route also send triggered partial updates, poisoning the failed route.

* Routers suspend split-horizon rules for the failed route by sending a poison reverse route back toward the router from which the poisoned route was learned.

* All routers place the route in holddown state and start a holddown timer for that route after learning that the route has failed.  Each router ignores all new information about this route until the holddown timer expires, unless that information comes from the same router that originally advertised the good route to that subnet.