RIP mainly uses the following three timers:
Update timer: triggers the sending of update packets every 30s.
Age timer: sets and keeps track of the 180-second time limit. If a RIP router does not receive an update packet from any of its neighbors within the aging time, the RIP router detects the route as unreachable.
Garbage-Collect timer: determines when to delete a packet entry. If the route is no longer valid after the timer expires, the entry is removed from the RIP routing table.
The three timers work together in the following way:
The advertisement of RIP routing update is triggered by the update timer every 30 seconds. Each entry is associated with the age timer and garbage-collect timer. When a route is learned and added in the routing table, the age timer is initialized. If no Update packet is received from the neighbor for 180 seconds, the metric value of the route is set to 16 (to specify the route as unreachable). At the same time, the garbage-collect timer is initialized. If no Update packet is received for 120 seconds, the entry is deleted after the garbage-collect timer expires.