RIP timers are the most basic thing in the world right? Even the command to set them is named timers basic… However in some documentation it is not really clear what the difference is between the invalid and holddown timer. The default timers are 30 for updates, 180 for invalid, 180 for holddown and 240 for flush. I have heard and seen described in official documentation that when a route is in holddown it will not accept routes with a worse metric but routes with a better metric. This is however not true. First lets describe the different timers.
Updates – Updates are sent every 30 seconds by default to the address 184.108.40.206.
Invalid – If there has not been any updates for 180 seconds about the prefix it is consider invalid and the route will be poisoned (route advertised with a metric of 16).
Holddown – The timer for holddown will be activated when the route goes into an invalid state. This is set to 180 by default.
Flush – This timer is set to 240 seconds, when a routes is 240 seconds old it is flushed from the routing table.
So the holddown timer is used to stabilize the topology, even better routes will be suppressed which is not what some documentation says. Here is how I tested it.
I created a topology with 3 routers connecting to each other and both the routers announced 220.127.116.11/32 to the middle router. I created an ACL on the middle router to filter all traffic so that the best route will become invalid. On the third router I used an offset-list to make the route worse. After the route became invalid I stopped sending the route with a worse metric and sent it with a better metric. However the route is still not installed until the holddown timer has expired. If you manipulate the timers it is easier to see. I used 5 seconds for updates, 30 for invalid, 30 for holddown and flush of 240. You will see that it takes 60s before the route gets installed.
If you use the standard timers the holddown timer will not expire before the route is flushed since the 180 seconds start counting after 180s by default and then there is only 60s left until the route is flushed. Try this out for yourself and see if you get the same results as I.
Here is a good link describing the timers.