Holddown timer explained
Another feature used by distance vector routing protocols (such as RIP) to prevent routing loops is the holddown timer. This feature prevents a router from learning new information about a failed route until the timer expires. Here is how it works:
1. a router receives an information that a route is unreachable and starts the holddown timer.
2. until the holddown timer expires (180 seconds by default in RIP), the router will disard any routing updates that indicate the route is reachable.
3. only routing updates about the failed route that will be processed are the ones sent by the same router that originally advertised the route.
Consider the following example:
We have a network of two routers. Both routers are running RIP and R2 has advertised the 10.0.0.0/24 network to R1. Consider what happens if the network fails:
1. R2 advertises the 10.0.0.0/24 network with the infinitive metric (16) to R1, indicating that the network is no longer accessible.
2. R1 receives the routing update, marks the route as unreachable, and starts the holddown timer.
3. During the holddown period, R1 will not process any routing update about that route received from other routers. Only updates from R2 will be processed:
Note that the IP address 192.168.0.2 listed above is the IP address of the interface on R2 that is directly connected with R1.