RIP General Operation, Messaging and Timers
(Page 2 of 3)
RIP Update Messaging and the 30-Second Timer
RIP Request messages are sent under special circumstances, when a router requires that it be provided with immediate routing information. The most common example of this is when a router is first powered on. After initializing, the router will typically send an RIP Request on its attached networks to ask for the latest information about routes from any neighboring routers. It is also possible for RIP Request messages to be used for diagnostic purposes.
A router receiving an RIP Request will process it and send an RIP Response containing either all of its routing table, or just the entries the Request asked for, as appropriate. Under normal circumstances, however, routers do not usually send RIP Request messages asking specifically for routing information. Instead, a special timer is used on each RIP router that goes off every 30 seconds. (This timer is not given a specific name in the RIP standards; it is just the 30 second timer).
Each time the timer expires, an unsolicited (unrequested) broadcast/multicast is made of an RIP Response message containing the router's entire routing table. The timer is then reset and 30 seconds later goes off again, causing another routine RIP Response to be sent. This process ensures that route information is regularly sent around the internet, so routers are always kept up to date about routes.
When a router receives routing information and enters it into its routing table, that information cannot be considered valid indefinitely. In our example in the previous topic, suppose that after Router B installs a route to Network 1 through Router A, the link to between RA and N2 fails. Once this happens, N1 is no longer reachable from RB, but RB has a route indicating it can reach Network 1.
To prevent this problem, routes are only kept in the routing table for a limited amount of time. A special Timeout timer is started whenever a route is installed in the routing table. Whenever the router receives another RIP Response with information about that route, the route is considered refreshed and its Timeout timer is reset. As long as the route continues to be refreshed, the timer will never expire.
If, however, RIP Responses containing that route stop arriving, the timer will eventually expire. When this happens, the route is marked for deletion, by setting the distance for the route to 16 (which you may recall is RIP infinity and indicates an unreachable network). The default value for the Timeout timer is usually 180 seconds. This allows several periodic updates of a route to be missed before a router will conclude that the route is no longer reachable.
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Version 3.0 - Version Date: September 20, 2005
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