Rip loop prevention methods 5(hold down timer)

Holddown is a loop-prevention mechanism employed by distance vector routing protocol. This feature prevents a router from learning new information about a failed route. When a router receives an information about an unreachable route, a holddown timer is started. The router ignores all routing updates for that route until the timer expires (by default, 180 seconds in RIP). Only updates allowed during that period are updates sent from the router that originally advertised the route. If that router advertise the update, the holddown timer is stopped and the routing information is processed.