Holddown

Holddown timer works by having each router start a timer when they first receive information about a network that is unreachable. Until the timer expires, the router will discard any subsequent route messages that indicate the route is in fact reachable.It can solve the case where multiple routers are connected indirectly. The classic example would be three routers configured in a triangle. In this situation,Split horizon and solit horizon with poisoned reverse can do nothing [The TCP/IP Guide-RIP Special Features For Resolving RIP Algorithm Problems ] .

In other words, a holddown keeps a router from receiving route until the network appears to be stable -- until either an interface stops changing state (flapping) or a better route is learned.

Holddowns are usually implemented with timers. If the router detects a network unreachable, the timer is started. The router will then wait a preset number of seconds until the network is stable. When the timer expires, the router will then receive its routing updates from other routers.