Holddown

The holddown timer is an algorithm that ensures the stability of routed computer networks.

If several network topologies interconnected, you need to branch router, which should determine the fastest or best route for a data packet through the network. In order to perform this operation efficiently, save router locally the topology of the network in a routing table.

If a power failure occurs, it is not immediately deleted from the table, but is for a certain time ( the "hold - down time" ) still left there, but as " not available" selected. This allows other routers can adjust it and the network comes back to a stable state. In addition, the formation of routing loops are prevented, because during the holddown time the router accepts no route with a higher metric than the route previously marked as unavailable. Only from the router, which has information about the failed route, each route is assumed that the failed route could be available again or an alternative path is found.