VLANs: What They Do and Why They Matter

8/7/2014 4:00:00 PM
As a network specialist, I get a lot of questions about VLANs, so I thought a refresher was in order.

The major functions of VLANs (Virtual Local Area Networks) on an IP network are to limit the size of broadcast domains, improve network performance, and bolster the level of security on the network.

In a nutshell, a VLAN is a logically separate IP sub-network that allows multiple IP networks and sub-networks to exist on the same-switch network. It is also a logical broadcast domain that can span multiple physical LAN segments. It is a modern way for administrators to configure switches to improve network performance by separating large broadcast domains into smaller ones.

By using VLANs, a network will be able to group together stations by logical function or by applications, without regard to the physical location of the users. Each VLAN functions as a separate LAN and spans one or more switches. This allows host devices to behave as if they were on the same network segment. When using VLANs, a Layer 3 device is required (a Layer 3 device refers to either a router or a Layer 3 switch).

A Real-World VLAN Scenario
Think about a client that owns a small home with a single zone of audio/video and automation. Some years later, she adds a second story to her home. The network is still the same, but the living space has doubled from 5,000 square feet to 10,000 square feet. This means that devices will move, more equipment will be put into the house, and now the single zone of audio/video and the automation cannot provide the needs of the customer. The homeowner had no issues with her network before because of the amount of data being transferred was very small. Now she will be adding a phone system, VoIP, IP cameras, multiple A/V zones, and more automation to her new theater room and other locations. Her network was able to handle the load before, but now the amount of data handled by the network has become more than five times the original amount.

This is where VLAN switching comes in. It will be easier to add the additional desired system for the client if things like automation, audio/video distribution, VoIP, and IP cameras can be separated into VLANs on a network by simply changing settings on a managed switch and router. This is where planning ahead comes in handy, choosing the right gear to future-proof projects. This will certainly make it easier to manage specific security and bandwidth needs. The admin can create groups of logically networked devices that act as if they are on their own independent network, even if they share a common infrastructure with other VLANs. Also with the right system, you can enable cross-VLAN communication that will allow you to have devices on one VLAN talk to another. These are policies typically programmed into the router and switch to allow multicast and uni-cast devices to talk across VLANs.
VLANs are meant to limit the size of broadcast domains, which means those broadcasts do not affect the rest of the network. (AppleTVs, printers, ZigBee, etc. are types of typical broadcast devices). When you have a large network, it is very important that you opt for using a switch and router with VLAN capabilities. Networks that have too much unsegmented data have issues with devices slowing down, devices not connecting, and even random equipment failure (devices get overloaded).

In Summary
  • VLANs are independent LAN networks.
  • VLANs allow devices to separate, although they share the same infrastructure.
  • VLANs cut down on broadcast storms, which can effect entire networks.
  • VLANs allow users to separate out large networks with lots of data, without loss of communication between certain devices (multicast, uni-cast devices)

Advantages of VLANs
  1. Security - Sensitive data can be separated from the rest of the network, decreasing the chances of confidential information breaches. (You can set up a guest network through wireless 801.1q VLAN tagging so that if you tag the wireless with a separate sub-network, others cannot access internal data from the guest wireless network.
  2. Higher Performance - Division of Layer 2 networks into multiple logical work groups (broadcast domains) reduces unnecessary traffic on the network and boosts performance.
  3. Cost Reduction - Cost savings result from less need for expensive network upgrades and less service calls required.

Jason Gibson is a computer enthusiast and network security and installation professional. 
Jason Gibson


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