The invalid route timer controls how long the router will wait before a particular route is declared invalid. If the route disappears from routing updates from neighboring routers for this length of time, the router will mark it invalid. The router will set the RIP metric for invalid routes to 16, or unreachable, when distributing them. This technique is often called "route poisoning." It is important to remember that the router doesn't remove invalid routes from its own routing table, and it will continue to distribute them to other routers.
The default invalid route timer value is 180 seconds. Most references advise making this value at least three times the update period. Shorter invalid route timers can cause instability problems. But if you make it too much longer, then the network doesn't respond well to topology changes.
The holddown timer for a particular route starts when the router gets a routing update that says that a particular route is inaccessible. This could happen, for example, if another router's invalid route timer for this route has expired. But it can also happen as a result of a triggered update, indicating that a particular interface pointing to this network has gone down.
The router will keep the unreachable route in its table, and will distribute it to other routers with an unreachable metric. After the holddown timer expires, the router will delete the unreachable route and start to accept other routing information for this route.
The default holddown period is 180 seconds. It is usually a good idea to keep the holddown time the same as the invalid route timer to help ensure network stability.
flush timer. This controls how long the router will keep a route in its routing table before purging it. The default flush period is 240 seconds. It must be greater than the holddown time; otherwise, routes can be flushed before the holddown timer expires, which makes the holddown time meaningless
Router2(config-router)#timers basic 20 80 80 120
The four arguments are, in order, the update period, the invalid route timer, the holddown timer, and the flush timer. All of these times are in seconds.
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