The switch knows that the frame came in port E0 and that E0 has
been configured as part of VLAN1. The switch looks only at the VLAN1 address
table and finds a match. So, the switch forwards the frame. Even if there had
not been a match in the VLAN1 address table, the switch would have flooded the
frame, but only out ports in VLAN1. Therefore, neither Wilma nor Betty could get
a copy of the frame.
In short, VLANs act just like physical LANs. The only
difference is that physical LANs include all physical ports on a switch, whereas
VLANs include a subset of the ports on a switch, based on the configuration that
the network engineer adds. Actually, you can take the list of facts about
physical LANs from earlier in this chapter, change the word "LAN" to "VLAN," and
they are all still true:
-
Each VLAN has an independent MAC address table as compared to
the other VLANs.
-
Broadcasts originating in one VLAN are flooded inside that
VLAN.
-
Broadcasts originating in one VLAN are not forwarded into the
other VLANs.
-
Unicasts originating in one VLAN are not forwarded into the
other VLANs.
That's all there is to VLANs. A VLAN is just a LAN, or
broadcast domain, that is created by configuring a switch. By telling the switch
to treat some ports as if they are in one LAN, and others as if they are in a
second LAN, you can create multiple virtual LANs.
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