Physical LANs: It's All About Broadcasts
Many people have different ideas about what a network really
is. For instance, if you ask someone who works on servers, that person might
think of a LAN as a server that's attached to an Ethernet. The person who runs
the Ethernet cables has a different perspective, thinking of the LAN as a bunch
of cables. However, the perspective you need to have to understand VLANs is
closer to that of the network engineer, who cares more about hubs and
switches.
As you probably recall, hubs and switches behave differently.
Hubs repeat all received electrical signals out all ports, except the port in
which the signal was received, essentially flooding the frame. Switches do not
flood all received frames, but instead use a MAC address table, which tells them
how to best forward a frame.
However, hubs and switches process broadcast frames
indentically. A broadcast frame is an Ethernet
frame that has a destination MAC address field set to FFFF.FFFF.FFFF. Broadcast
frames are sent for a variety of reasons. (They are particularly useful when a
computer doesn't know some piece of information; the computer can send a
broadcast to everyone, asking for that tidbit of information.) Regardless of why
the broadcast frame is sent, when a switch receives a frame whose destination
address is the Ethernet broadcast address, it forwards the frame out all ports
except the incoming port. A hub repeats all framesincluding broadcastsout all
ports except the incoming port. (In fact, hubs do not even look at the
addresses.) So, although the internal processing on switches and hubs is
different, essentially a switch acts just like a hub in regards to broadcast
frames.
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