RIP General Operation, Messaging and Timers
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Removing Stale Information: The Garbage-Collection Timer
When a route is marked for deletion, a new Garbage-Collection timer is also started. Garbage collection is a computer industry phrase for a task that looks for deleted or invalid information and cleans it up. Thus, this is a timer that counts a number of seconds before the newly invalid route will be actually removed from the table. The default value for this timer is 120 seconds.
The reason for using this two-stage removal method is to give the router that declared the route no longer reachable a chance to propagate this information to other routers. Until the Garbage-Collection timer expires, the router will include that route, with the unreachable metric of 16 hops, in its own RIP Responses, so other routers are informed of the problem with that route. When the timer expires the route is deleted. If during the garbage collection period a new RIP Response for the route is received, then as you might expect the deletion process is aborted: the Garbage-Collection timer is cleared, the route is marked as valid again, and a new Timeout timer starts.
In addition to the two situations already described where an RIP Response is sentin reply to an RIP Request and on expiration of the 30-second timeran RIP Response is also sent out when a route changes. This action, an enhancement to basic RIP operation called a triggered update, is intended to ensure that information about route changes is propagated as fast as possible across the internetwork, to help reduce the slow convergence problem in RIP. For example, in the case we just saw where a route timed out and the Garbage-Collection timer was started, a triggered update would be sent out about the now-invalid route immediately. This is described in more detail in the topic on RIP special features.
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