If the router does not hear from its neighbor for 180 seconds, the router assumes that the neighbor is down or that the link connecting the two neighbors is down. At this point, the route is marked as invalid. An invalid timer is used to determine that no fresh information is received about this route. This timer is set to 180 seconds for RIP. Upon expiration of the invalid timer, hold-down time begins. During the hold-down period, the route is marked as possibly down and the metric is set to infinity. In addition, the route is advertised to the router's neighbors with an infinity metric. When the hold-down timer expires, a request is sent to query neighbors for an alternate route to the destination. If a new route was received during the invalid or hold-down period, the router begins advertising this new route.
The last item is the flush timer, which begins immediately after the invalid timer expires, and lasts 60 seconds after the hold-down expires. Upon the expiration of the flush timer (240 seconds), the route is deleted if no replacement route is received. The advantage of these timers is that you can determine whether the route was not received because of some transient condition, or that the route has actually been removed. This ensures that if an interface has flapped, you can still converge to accommodate the flap. The disadvantage, however, is slow convergence. As routers and links are becoming faster, network administrators want faster convergence, rather than waiting for a protocol to install an alternate path after a long period of time.
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