The Holddown Process and Holddown Timer

The holddown process is a mechanism used by routing protocols to avoid the propagation of incorrect routing information due to temporary inconsistencies in the network. When a network topology change occurs, routers exchange routing updates to inform each other about the changes. During this exchange, a router might receive information from multiple sources that conflict with each other. This can happen if a router receives an update from a neighbor that claims a network is unreachable, while at the same time receiving another update from a different neighbor that claims the same network is reachable.

In order to prevent the propagation of incorrect information, routers implement the holddown process. When a router receives an update that claims a network is unreachable, it places that network in a holddown state. During the holddown state, the router will not accept any updates about that network from any source, and it will not advertise that network to any other routers. The holddown timer specifies the duration of the holddown state.

The purpose of the holddown timer is to allow time for all routers to converge on the correct routing information before allowing any further updates. Once the holddown timer expires, the router removes the network from the holddown state and accepts updates about that network again. This helps to prevent the creation of routing loops and other inconsistencies in the network.

Holddown timers are typically implemented in distance-vector routing protocols such as Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP). The exact implementation of the holddown timer may vary depending on the specific routing protocol being used.