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802.1Q VLAN User’s Guide 1-1
CHAPTER 1
VIRTUAL LOCAL AREA NETWORKS
This chapter introduces the concepts of Virtual Local Area Networks
(VLANs) and discusses the central concepts of IEEE 802.1Q VLANs.
This chapter also contains information on how to contact Cabletron
Systems for additional support related to VLANs.
1.1 DEFINING VLANs
A Virtual Local Area Network is a group of devices that function as a
single Local Area Network segment (broadcast domain). The devices that
make up a particular VLAN may be widely separated, both by geography
and location in the network.
The creation of VLANs allows users located in separate areas or
connected to separate ports to belong to a single VLAN group. Users that
are assigned to such a group will send and receive broadcast and multicast
traffic as though they were all connected to a common network. VLAN
aware switches isolate broadcast, multicast, and unknown traffic received
from VLAN groups, so that traffic from stations in a VLAN are confined
to that VLAN.
When stations are assigned to a VLAN, the performance of their network
connection is not changed. Stations connected to switched ports do not
sacrifice the performance of the dedicated switched link to participate in
the VLAN. As a VLAN is not a physical location, but a membership, the
network switches determine VLAN membership by associating a VLAN
with a particular port or frame type.
Figure 1-1 shows a simple example of a port based VLAN. Two buildings
house the Sales and Finance departments of a single company, and each
building has its own internal network. The stations in each building
connect to a SmartSwitch in the basement. The two SmartSwitches are
connected to one another with a high speed link.