RIP Timers

RIP uses numerous timers to regulate its performance. Following are the timer stages for RIP:

  • Update—The routing-update timer is the interval between periodic routing updates. This is how often the device sends routing updates. Generally, it is set to 30 seconds, with a small random amount of time added whenever the timer is reset. This is done to help prevent congestion, which could result from all routers simultaneously attempting to update their neighbors.

  • Invalid—Each routing table entry has a route-timeout timer associated with it. This is the number of seconds since the device received the last valid update. When the route-timeout timer expires, the route is marked invalid but is retained in the table until the route-flush timer expires. Once this timer expires, the route goes into holddown. The default is 180 seconds (3 minutes).

  • Holddown—The holddown period is the number of seconds the system waits before accepting any new updates for the route that is in holddown (that is, routes that have been marked invalid). The default is 180 seconds (3 minutes).

  • Flush—The route-flush timer is the number of seconds since the system received the last valid update until the route is discarded and removed from the routing table. The default is 240 seconds (4 minutes).

As an example, when the interface on an adjacent router goes down, the system no longer receives routing updates from the adjacent router. At this time, the Invalid and Flush timers start increasing. In the first 180 seconds, nothing will happen. After 180 seconds, the invalid timer expires, making the route invalid, and the Holddown timer starts and holds the route for another 60 seconds. If there is still no update regarding the interface status on the adjacent router (that is, it is still down), then the route enters into the Flush state where in total the system has waited for 240 seconds from the last update (180 seconds for the Invalid timer and 60 seconds for Holddown timer), and the system flushes the route. Even if the adjacent routers interface comes up immediately, the system does not accept a routing update until the Holddown timer completes the remaining 120 seconds.