The holddown timer activates when a route is marked down. Until the timer expires, the router does not accept any new information about that route. This is very useful if you have a flapping route because it will prevent your router from sending out updates and being part of the problem in flooding the network. The potential down side is if the route comes back up while the timer has not expired, that route will be unavailable for that period of time. This is only a problem if this is a major route used by the majority of your traffic. Otherwise, this is a minor problem as traffic can be re-routed around the outage.