Update:how often the update for a specific route must be sent. (30 seconds)
Invalid: states that once an update is received for a route how long to wait until the route is considered invalid or non-reachable (180 seconds)
Hold Down: If a route gets a worse metric and is updated, the hold down timer is started. During this time no other metric for the route is accepted which is equal or worse than the current metric (180 seconds)
Flush: how many seconds, since the last valid update, until delete route (240 seconds)
invalid and Flush timer will start/reset every time an update comes in.
When you get an update the Invalid Timer starts to count to 180 and the Flush Timer to 240 seconds. They start at the same time!
After 180 seconds of the Invalid Timer have passed, the Holddown Timer will start for 60 seconds (Until the route is flushed). When this Holddown Timer run for 60 seconds it will ignore any (even better) routes it get advertised with, to give the network to properly converge/stabilize.
For more info http://blog.ine.com/2010/04/15/how-basic-are-rip-timers-test-your-knowledge-now/
