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A hold down timer is a technique used by routers. When a router receives a notification about an offline route or node, the router will initiate a hold down timer allowing the offline router to recover and not update its routing table until the time expires.
The router starts a hold down timer as soon as it receives an update that a specific network, node, path or router is unreachable. During this time it will not accept any update from the neighbors falling on that route.
Techopedia Explains Hold Down Timer
An internetwork, Internet or autonomous system may consist of many different nodes, networks and intermediary devices like routers and switches. Routers maintain paths to all the nodes and networks in its routing table, continuously updating the optimal path whenever it receives a new or better path from its neighboring routers.
If a router receives a notification that a specific path or network has gone offline, the router starts a timer and discards all the updates it gets from the neighboring routers. The primary objective for this technique is to allow the node to regain its online status and discard updates still advertising the old and inferior route. By initiating the hold down timer, a router can prevent itself and the network from a possible routing loop. Normally the hold down time is 180 seconds in routing information protocols.
