RIP Timers

RIP Timers - Overview

RIP does not have any way of reliability therefore the updates are sent periodically. RIP can also configured for generating triggered updates, however this is not discussed in this article.  RIP uses 4 timers in order to manage the advertisement of updates.

RIP Timers

  • RIP Timers - Update Timer: It is the  time between each update interval. By default this value is set to 30 seconds. We can also change this timer using configuration. 

  • RIP Timers - Invalid Timer: The time after which a suspect route becomes invalid. This is set to 180 seconds, by default and is configurable too.  Invalid timer starts at same time as update timer starts, for example, when a router R1 receives update a prefix 'X' from a neighbor R2 at T+0 seconds, it expects another update from R2 for the same prefix at T+31 seconds. If R1 doesn't receive an update for prefix 'X' for another 150 seconds i.e. T+180 seconds(30 +150), the prefix is declared Invalid. This means that after total 180seconds or after Invalid timer expiry, R1 will set the hop count of the routing entry of the prefix to 16(infinite metric/hop count for RIP), marking the destination as unreachable. In another words, Prefix 'X' is called a poisoned route.

  • RIP Timers - Flush Timer: Flush timer starts at same time as update timer starts. After waiting for T+180 seconds R1 declared the prefix 'X' Invalid. After waiting for another 60 seconds (180+60 = 240seconds), the prefix is flushed out or removed from the routing table.

  • RIP Timers - Holdown Timer:  It is the time used to suppress the possibility of defective routes being installed in the routing table. The default time is 180 seconds.After no updates for 180 seconds the route becomes Invalid, the Holddown timer starts at this time. So the Holddown timer never finishes because it would have to run for 360 seconds before leaving Holddown, 180 + 180. The only way to test behavior of Holddown is to set timers to something different. So with the timers basic command you could set it to 10 30 30 180. The route would become invalid after 30 seconds and and enter Holddown, after 30 more it could leave Holddown. If nothing happens for 180 seconds in total the route will be flushed. The Holddown timer is used to stabilize the network and to prevent loops too. When a route enters Holddown, it can't be installed even if there is a new route with a better metric. Let's assume prefix 'X' comes back up and R2 started advertising it back to R1. Let's assume the update is received when R1 is already running holdown timer for prefix 'X'. R1 will not accept this prefix in routing table unless Holddown timer exp

  • Reference Commands:
          R1#show ip protocols | include rip|seconds
          Routing Protocol is "rip"
          Sending updates every 30 seconds, next due in 23 seconds
          Invalid after 180 seconds, hold down 180, flushed after 240
          Redistributing: rip

          R1#debug ip rip
          RIP protocol debugging is on

  • Changing RIP Timers:  RIP default timers are changed for troubleshooting purpose or debug purpose. using below command-line, we have changed the update timer, Invalid timer, Holddown timer and Flush timer to 10seconds, 30seconds, 30seconds, and 180 seconds respectively.
           R1(config)#router rip
           R1(config-router)#timers basic 10 30 30 180

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