Why a route is flushed after 60s ???

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Mar 8th, 2009
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Every 30seconds, a router receives from one of its neighbors RIP updates for a specific route. Suddenly that route gets down and then the router received immediately triggered update from that neighbor for that route, now with metric=16. The route is immediately removed from its routing table, though for the next 60 seconds it keeps advertising this route to all its neighbors, with metric=16. After that it stops advertising the route.

Any idea where those 60seconds are coming from???

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jorge.calvo Tue, 03/10/2009 - 17:30
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This is the default RIP behavior.

RIP hello time is 30 seconds. If a hello is not received from the neighbor after 180 seconds (invalid timer)it advertises it with 'infinite' metric, 16 in RIP case. This way it advice its neighbors not to use that 'dead' route.

The 60 seconds timer is called the 'flush timer' or the 'garbage collector timer' and it is used by RIP only as a routing table cleaning tool.

Another advantage is that the route will remain in the routing table after the flush timer expires, that way if a new hello is received, the route still will be on the table.


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