Understand RIP Routing Timers All in One Shot!

Cisco’s RIP implementation defines 4 times tight to one periodic update interval and 3 states: invalid, hold-down and flush.

The misleading information is that they are referred to as “timers” in configuration part!

Apart from the periodic update interval the other times correspond to exactly 2 timers that are attached to the route prefix and to path descriptors respectively.

It is to note that every network route in routing table is coded as a prefix descriptor that matches one or many path descriptors.

To a path descriptor we associate the invalid timer so that a route path is invalidated after the invalid time has expired.

Then to each prefix we associate only ONE timer that moves the prefix to garbage state after holddown state, and removes it from the routing table after garbage time has elapsed.

It is to note that:

  • Holddown and garbage timer runs separately from invalidation path timer;
  • Only garbage time is a specification of the 1058 RFC;
  • Each prefix can be in one of this states: valid, holddown, or garbage;
  • A prefix in holddown state can not be updated!

The 2 timers are reset every time an update is received which is different from the peridioc interval that triggers an update each time this constant time interval has fired.


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