Timers

RIP uses the following timers:

• Update timer: The Update timer periodically triggers Update packet transmission. By default, the interval at which Update packets are sent is 30s.

• Age timer: If a RIP device does not receive any packets from its neighbor to update a route before the route expires, the RIP device considers the route unreachable. By default, the age timer interval is 180s.

• Garbage-collect timer: If a route becomes invalid after the age timer expires or a route unreachable message is received, the route is placed into a garbage queue instead of being immediately deleted from the RIP routing table. The garbage-collect timer monitors the garbage queue and deletes expired routes. If an Update packet of a route is received before the garbage-collect timer expires, the route is placed back into the age queue. The garbage-collect timer is set to avoid route flapping. By default, the garbage collect timer interval is 120s.

• Hold-down timer: If a RIP device receives an updated route with cost 16 from a neighbor, the route enters the holddown state, and the hold-down timer is started. To avoid route flapping, the RIP device does not accept any updated routes until the hold-down timer expires, even if the cost is less than 16 except in the following scenarios:

  1. The cost carried in the Update packet is less than or equal to that carried in the last update packet.
  2. The hold-down timer expires, and the corresponding route enters the Garbage state.

The relationship between RIP routes and the four timers is as follows:

• The advertisement of RIP routing updates is triggered by the update timer with a default value 30 seconds.
• Each routing entry is associated with two timers: the age timer and garbage-collect timer.
  1. Each time a route is learned and added to the routing table, the age timer is started.
  2. If no Update packet is received from the neighbor within 180 seconds after the age timer is started, the metric of the corresponding route is set to 16, and the garbage-collect timer is started.
• If no Update packet is received within 120 seconds after the garbage-collect timer is started, the corresponding routing entry is deleted from the routing table after the garbage-collect timer expires.
• By default, the hold-down timer is disabled. If you configure a hold-down timer, it starts after the system receives a route with a cost greater than 16 from its neighbor.