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Switching Services
85
Loop Avoidance
Redundant links between switches are a good idea because they help prevent
complete network failures in the event one link stops working.
Sounds great, but even though redundant links can be extremely helpful,
they often cause more problems than they solve. This is because frames can
be broadcast down all redundant links simultaneously, creating network
loops as well as other evils. Here's a list of some of the ugliest problems:
If no loop avoidance schemes are put in place, the switches will flood
broadcasts endlessly throughout the internetwork. This is sometimes
referred to as a
broadcast storm
. (But most of the time it's referred to in
ways we're not permitted to repeat in print!) Figure 2.6 illustrates how
a broadcast can be propagated throughout the network. Observe
how a frame is continually being broadcast through the internetwork's
Physical network media.
F I G U R E 2 . 6
Broadcast storm
A device can receive multiple copies of the same frame, since that
frame can arrive from different segments at the same time. Figure 2.7
demonstrates how a whole bunch of frames can arrive from multiple
segments simultaneously. The server in the figure sends a unicast
(which is a directed broadcast) frame to Router C. Since it's a type of
broadcast, Switch A forwards the frame, and Switch B provides the
same service--it forwards the broadcast. This is bad because it means
that Router C receives that unicast frame twice, causing additional
overhead on the network.
Segment 1
Segment 2
Broadcast
Switch A
Switch B
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