(metallurgy) A device that holds the outer portion of a metal sheet in place during deep-drawing operations in order to keep it from becoming wrinkled.
(petroleum engineering) A device to anchor an oil well rod pump in its position.
Holddown
| Sci-Tech Dictionary: holddown |
(′hōl′dau̇n)
| 5min Related Video: Holddown |
| Computer Desktop Encyclopedia: holddown timer |
A function that prevents a router from being updated for a specified period in order to give other nodes some time to reconfigure and prevent a routing loop. When a router is notified of a route failure, it starts the holddown timer. In the meantime, if a notification of a route is received from its neighbor with equal or better metrics than the route that failed, the router stops the timer and updates its routing table. If the new route metrics are inferior, it keeps the timer running and does not update. See routing loop and route poisoning.
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| Wikipedia: Holddown |
This article is about the network routing heuristic. For other uses, see Holddown (disambiguation).
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 split horizon with poisoned reverse can do nothing[1].
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. For examle in RIP the default holddown timer is setting on 180 seconds.
References
- ^ The TCP/IP Guide-RIP Special Features For Resolving RIP Algorithm Problems
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 | route poisoning (technology) |
| Extreme Discovery Protocol |
| Convergence (routing protocol) |
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