As networks have grown in size and complexity, many companies have turned to virtual local area networks (VLANs) to provide some way of structuring this growth logically. Basically, a VLAN is a collection of nodes that are grouped together in a single broadcast domain that is based on something other than physical location.
You learned about broadcasts earlier, and how a router does not pass along broadcasts. A broadcast domain is a network (or portion of a network) that will receive a broadcast packet from any node located within that network. In a typical network, everything on the same side of the router is all part of the same broadcast domain. A switch that you have implemented VLANs on has multiple broadcast domains, similar to a router. But you still need a router (or Layer 3 routing engine) to route from one VLAN to another -- the switch can't do this by itself.
Here are some common reasons why a company might have VLANs:
- Security - Separating systems that have sensitive data from the rest of the network decreases the chances that people will gain access to information they are not authorized to see.
- Projects/Special applications - Managing a project or working with a specialized application can be simplified by the use of a VLAN that brings all of the required nodes together.
- Performance/Bandwidth - Careful monitoring of network use allows the network administrator to create VLANs that reduce the number of router hops and increase the apparent bandwidth for network users.
- Broadcasts/Traffic flow - Since a principle element of a VLAN is the fact that it does not pass broadcast traffic to nodes that are not part of the VLAN, it automatically reduces broadcasts. Access lists provide the network administrator with a way to control who sees what network traffic. An access list is a table the network administrator creates that lists which addresses have access to that network.
- Departments/Specific job types - Companies may want VLANs set up for departments that are heavy network users (such as multimedia or engineering), or a VLAN across departments that is dedicated to specific types of employees (such as managers or sales people).
You can create a VLAN using most switches simply by logging into the switch via Telnet and entering the parameters for the VLAN (name, domain and port assignments). After you have created the VLAN, any network segments connected to the assigned ports will become part of that VLAN.
While you can have more than one VLAN on a switch, they cannot communicate directly with one another on that switch. If they could, it would defeat the purpose of having a VLAN, which is to isolate a part of the network. Communication between VLANs requires the use of a router.
VLANs can span multiple switches, and you can have more than one VLAN on each switch. For multiple VLANs on multiple switches to be able to communicate via a single link between the switches, you must use a process called trunking -- trunking is the technology that allows information from multiple VLANs to be carried over a single link between switches.
On the next page, you'll learn about trunking.