Hi Hussein,
I did some labbing / debugging to verify how it exactly works, the RIP timers sound simple but there’s some stuff going on. Let’s say we have two routers:
R1-R2
R1 is advertising 1.1.1.0 /24 to R2.
Suddenly, R1 stops advertising RIP updates to R2.
At this moment, the invalid timer (180 seconds) and flush timer (240 seconds) start counting.
After 180 seconds, the route is considered invalid and the holddown timer will start (180 seconds). The route will remain in the routing table but is “possibly down”.
60 seconds later, the flush timer will expire and the route will be deleted from the routing table…that’s it. In total it takes 240 seconds for a route to be removed. The thing that confused me is that the holddown timer only runs for 60 seconds in this scenario, once the flush timer is expired it’s game over…
Apparatently, the holddown timer is also supposed to start when you receive a triggered update from a neighbor that advertises a route that is unreachable. The confusing part is that it doesn’t work this way on newer IOS versions. Once you receive a triggered update with a network that is unreachable, it’s removed from your routing table immediately. In some of the discussions I read online they say that on IOS 12.0, RIP does enter the holddown state when you receive a triggered update with an unreachable network.
I hope this helps, I’ll publish my debugs tomorrow.
Rene