Using VLANs in Your Network

A VLAN is a Virtual Local Area Network — a grouping of network devices that is logically segmented by functions, project teams, or applications without regard to the physical location of users. For example, several end stations might be grouped as a department, such as Engineering or Finance, having the same attributes as a LAN, even though they are not all on the same physical LAN segment.

To accomplish this logical grouping, the network administrator uses 802.1Q VLAN-capable switching devices and assigns each switch port in a particular group to a VLAN. Ports in a VLAN share broadcast traffic and belong to the same broadcast domain. Broadcast traffic in one VLAN is not transmitted outside that VLAN.

Virtual LANs allow you to partition network traffic into logical groups and control the flow of that traffic through the network. Once the traffic and, in effect, the users creating the traffic, are assigned to a VLAN, then broadcast and multicast traffic is contained within the VLAN and users can be allowed or denied access to any of the network‘s resources. Also, you have the option of configuring some or all of the ports on a device to allow frames received with a particular VLAN ID and protocol to be transmitted on a limited number of ports. This keeps the traffic associated with a particular VLAN and protocol isolated from the other parts of the network.

The primary benefit of 802.1Q VLAN technology is that it allows you to localize and segregate traffic, improving your administrative efficiency, and enhancing your network security and performance.

VLAN Business Scenario shows a simple example of using port-based VLANs to achieve these benefits. In this example, two buildings house the Sales and Finance departments of a single company, and each building has its own internal network. The end stations in each building connect to a switch on the bottom floor. The two switches are connected to one another with a high speed link.

VLAN Business Scenario

Without any VLANs configured, the entire network in the example in VLAN Business Scenario would be a broadcast domain, and the switches would 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 Switch A, cross the high speed link to Switch B, and then be propagated out all switch ports on Switch B. The switches treat each port as being equivalent to any other port, and have no understanding of the departmental memberships of each workstation.

Once Sales and Finance are placed on two separate VLANs, each switch understands that certain individual ports or frames 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 Switch A, be sent to the ports connected to other local members of the Sales VLAN, cross the high speed link to Switch B, and then be sent to any other ports and workstations on Switch B that are members of the Sales VLAN. Separate VLANs also provides unicast separation between Sales and Finance. Finance can not ping Sales unless there is a routed VLAN configured for both Finance and Sales.

Another benefit to VLAN use in the preceding example would be your ability to leverage existing investments in time and equipment during company reorganization. If, for instance, the Finance users change location but remain in the same VLAN connected to the same switch port, their network addresses do not change, and switch and router configuration is left intact.