What are the RIP timers, and what does each one do?
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Say this first
Cisco defaults are update 30 seconds, invalid 180, holddown 180 and flush 240. Update is how often the full table is sent. If no update refreshes a route for 180 seconds it becomes invalid: it is marked unreachable but kept and advertised as poisoned. Holddown makes the router ignore worse information about a failed route for a while, so old news cannot bring it back. Flush removes the route 240 seconds after the last update.
Go deeper
Seeing them
R1# show ip protocols | include seconds|Invalid
Sending updates every 30 seconds, next due in 12 seconds
Invalid after 180 seconds, hold down 180, flushed after 240Change them all at once (and on every router) with:
router rip
timers basic 30 180 180 240A failure, minute by minute
- R2 stops hearing about 10.4.0.0/24 from R4 because the link failed silently (no interface down).
- At 180 seconds after the last update, R2 marks the route invalid, metric 16, and starts holddown.
- R2 advertises the route as 16 to its neighbors so they poison it too.
- At 240 seconds after the last update, R2 flushes it.
If the interface itself goes down, R2 does not wait: it poisons the route at once and sends a triggered update. The timers matter when failures are silent.
Why it is slow
A silent failure takes three minutes before anyone reacts. That is the main reason RIP is out of place anywhere convergence matters.
They will ask next
Why does IOS not send updates exactly every 30 seconds?
It adds a small random offset each time. If every router sent at the same moment, updates would synchronize and cause bursts of CPU and link load across the network.
Traps
- Mixing up invalid and flush. Invalid marks the route unreachable; flush deletes it.