route that might have gone bad. When a router goes down, neighboring routers detect this via
the lack of regularly scheduled update messages. These routers then calculate new routes and
send routing update messages to inform their neighbors of the route change. This activity begins
a wave of triggered updates that filter through the network. These triggered updates do not
instantly arrive at every network device. So it is possible for a device that hasn't been informed
of the change to send an update message with information stating that the route is still good to
a device that already knows about the failure.
time. The hold-down period usually is calculated to be just greater than the period of time
necessary to update the entire network with a routing change. Figure 6-4 shows an example of
a network in holddown.