The Holddown mechanism is used to delay processing updates about a network whose reachability has become questionable, as the received updates might not yet contain up to date information. Assume that a router suddenly stops receiving updates for a network from its only next hop neighbor to that network. It has not yet been declared unreachable, instead updates from that next hop are simply lacking the vector information for that specific network. This can happen for various reasons including:
- Update might have been lost in transit or dropped
- Next hop router might be turned off or crashed without the link going down
- Next hop router might have started considering us as its own next hop and split horizon is taking effect
- RIPv2 process may have been removed on the next hop router
- Address summarization, route filtering, or passive interface may have been configured
- next hop router might be running RIPv2 that does not support Route poisoning, so when a network goes down it simply no longer sends the network information in updates.
No matter what lack of information about a network from the next hop is ok for a short amount of time, but once that time is exceeded the router knows something happened to that network. Once its determined this the router cannot accept updates from any neighbors for this network in an effort to not cause routing loops. instead it gives other devices a certain amount of time to learn about the outage themselves and converge on a different path. This is what the holddown timer is used for.
Cisco RIPv2 implement the Invalid After timer to reset updates about specific networks and begins after it stops receiving updates about that network. After the Invalid after timer runs out and invalidates the route, the Holddown timer begins and forces neighbors to find an alternative route by advertising the network with an infinite metric and locks the routing entry in the table so it cannot be updated during this time.
After the holddown timer expires the router unlocks the routing entry and converges through a neighbor that offers the lowest metric route to the network like usual.