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That's because, so far, the logic defined depended upon a
gateway notifying its neighbors of whether its metrics had changed.
If the gateway could no longer communicate, it could not notify its
neighbors of a change. To handle such situations, RIP employs
timers: It sends out messages (every 30 seconds by default), and the
protocol assumes that if a gateway or specific route is not heard
from within 180 seconds, it no longer is available. Once it
determines that a route is unavailable, the router sends a special
message that notifies its neighbors of the unavailable route. After
270 seconds, if nothing is heard from the gateways or route, this
information is flushed from the router's routing table. These timers
are known as the update,
invalid,
and flush
timers, respectively. |
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