Handling of outdated routes in RIP Protocol

According to the V-D algorithm, a route is refreshed only when a better route appears; otherwise, it is retained in the routing database. This ignores the situation that when a route suddenly crashes, you need to select a new route to replace the existing route. But it is not solved by refreshing algorithm in V-D.

In practice, RIP specifies that all machines set a clock for each table in the route database. Each time a new table is added, set a new clock accordingly. When you receive the V-D message, if there is a table about this route, the clock is cleared and re-recorded. If the refresh information of the route has not been received within the specified time period and the clock expires, the route will be deleted from the route database.

If there are other routes to the specified destination, the new route will be obtained from the scheduled refresh message received. Otherwise, the route to the original destination does not exist.

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