VIRTUAL LOCAL AREA NETWORKS

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 single network segment.

VLAN aware switches isolate broadcast and multicast traffic received

from VLAN groups, keeping broadcasts from stations in a VLAN

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.

Pincture 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.

In this example, the Sales and Finance workstations have been placed on

two separate VLANs. In a plain Ethernet environment, the entire network

is a broadcast domain, and the SmartSwitches follow the IEEE 802.1d

bridging specification to send data between stations. A broadcast or

multicast transmission from a Sales workstation in Building One would

propagate to all the switch ports on SmartSwitch A, cross the high speed

link to SmartSwitch B, and be propagated to all the switch ports on

SmartSwitch B. The SmartSwitches treat each port as being equivalent to

any other port, and have no understanding of the departmental

memberships of each workstation.

In a port based VLAN environment, each SmartSwitch understands that

certain individual ports are members of separate workgroups. In this

environment, a broadcast or multicast data transmission from one of the

Sales stations in Building One would reach SmartSwitch A, be sent to the

ports connected to other local members of the Sales VLAN, cross the high

speed link to SmartSwitch B, and then be sent to any other ports and

workstations on SmartSwitch B that are members of the Sales VLAN.