Workshop

Switch Theory
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When I was a boy, we didn’t have any of this fancy switching nonsense. We walked uphill in the snow
both directions to our networks and used bridges to connect them, and we liked it!
Once upon a time, smaller switches were called bridges because they bridged two networks together.
Today’s switches can connect dozens of segments, but they still follow the rules that were invented for
their smaller forebears.
Many switches are now used as replacement hubs—shared media becomes very annoying if you have
many shared media errors (such as collisions), and you don’t get the full capacity of the pipe. As such,
they don’t speak network protocols the way that routers do.
A switch operates by learning the MAC addresses of each station attached to it. If one MAC address on
one segment wants to talk to another MAC address on another segment, the switch connects them
together on a private circuit, much like a switchboard operator of old would connect various cables and
plugs to appropriate outlets in order to connect various people’s conversations together. However, unlike
the switchboard operator, the switch could care less about the content of the conversation; it just blithely
connects folks who want to talk to each other and separates out the other conversations. This separation
of conversations is why folks use switching as a way to speed up their networks.
Experiencing strange problems on your large switched network? Check out how many simultaneous MAC
addresses your switches support—older switches didn’t support very many and won’t be able to keep
large numbers of end stations, which can manifest itself in PC lockups and disconnects.
Does each end station need to know that a switch is in the mix? Nope. This is because the switch will
respond when one end station’s calls for the other. In other words, Station A in Figure 14.4 wants to talk
to Station B, which is on the other side of the switch. Station B never sees the switch; instead, it sees the
unmodified frame from Station A, which the switch has lobbed from one segment to another. The frame
has a source of A and a destination of B; the switch’s MAC addresses never enter the picture. Scary, yet
very neat.