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In large networks of this type, new problems can arise. Late collisions occur when two
stations successfully test for a clear network and then transmit, only to encounter a
collision. This condition can occur when the network is so large that the propagation
of a transmitted frame from one end of the network to the other takes longer than the
test used to detect whether the network is clear.
One of the other major problems when using hubs is the possibility of broadcast
storms. Figure 2-6 shows two hubs connected with two connections. A frame enters
the network on Hub 1 and is replicated on every port, which includes the two connections
to Hub 2, which now repeats the frame out all of its ports, including the two ports
connecting the two switches. Once Hub 1 receives the frame, it again repeats it out
every interface, effectively causing an endless loop.
Figure 2-6. Broadcast storm
Anyone who’s ever lived through a broadcast storm on a live network knows how much
fun it can be—especially if you consider your boss screaming at you to be fun. It’s extra
special fun when your boss’s boss joins in. Symptoms of a broadcast storm include
every device essentially being unable to send any frames on the network due to constant
network traffic, all status lights on the hubs staying on constantly instead of blinking
normally, and (perhaps most importantly) senior executives threatening you with bodily
harm.
The only way to resolve a broadcast storm is to break the loop. Shutting down and
restarting the network devices will just start the cycle again. Because hubs are not generally
manageable, it can be quite a challenge to find a Layer-2 loop in a crisis.
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