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Holddowns are the method by which routing protocols prevent unstable network conditions from causing unnecessary or premature route propagation.
In other words, a holddown keeps a router from announcing route changes 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 change, 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 send out its routing updates to other routers.
The definition of network stability can vary. If a route becomes unavailable but another, more attractive route is announced, this is often interpreted as a return to stability, causing the timer to expire.
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