RIP Timers

The invalid timer is used to limit the amount of time a route can stay in a routing table without being updated. RIP calls this the expiration timer, or timeout. Cisco’s IOS calls it the invalid timer. The invalid timer is set to 180 seconds whenever a new route is established and is reset to the initial value whenever an update is heard for that route. If an update for a route is not hard within that 180seconds (six update periods), the hop count for the route is changed to 16, marking the route as unreachable.

Another timer, the flush (garbage collection) timer, is set to 240 seconds – 60 seconds longer than the expiration time. The route will be advertised with the unreachable metric until the flush timer expires, at which time the route is removed from the routing table. When a route has been marked as unreachable, but has not yet been flushed, it will show the route in the routing table as,possibly down.

The third timer is the holddown timer. Although RFC1058 does not call for the use of holddown’s, Cisco’s implementation of RIP does use them. An update with a hop count higher than the metric recorded in the routing table will cause the route to go into hold-down for 180 seconds (again, six update periods).

This information comes from Jeff Doyle's Routing TCP/IP Volume I book which I highly recommend to you. Happy Reading!