VLAN - Benefits and Limitations

Introduction

The purpose of this walkthrough is to provide a better understanding of Virtual Local Area Networks and their use in the networking world. In the following sections, I will define a VLAN and describe its benefits as well as some limitations. I will explain why it is important for a network administrator to understand VLANs, and give some basic instructions to determine how many VLANs a department would typically need. Finally, contact information will be provided for any additional questions you might have.

Use of this information will be used to determine a department's VLAN needs. If VLANs are well thought out in advance, the need to readdress devices and modify VLAN configurations more than once will not become an issue. This will save everyone involved a great deal of effort and minimize the amount of changes that will be needed.

What is a VLAN?

Virtual LANs can be viewed as a group of devices on different LAN segments which can communicate with each other as if they were all on the same physical LAN segment.

Switches using VLANs create the same division of the network into separate domains but do not have the latency problems of a router. Switches are also a more cost-effective solution.

By now you are probably wondering why someone would go to all this work to end up with what appears to be the same network as the original one. In many instances, LANs have been grouped with physical location being the primary concern. VLANs are built with traffic patterns in mind. Using VLANs, we are able to group these devices into a single domain. This allows us to confine broadcast traffic for this workgroup to just those devices that need to see it, and reduce traffic to the rest of the network. There is an increased connection speed due to the elimination of latency from router connections. An additional benefit of increased security could be made if you make the decision to not allow access to the host from foreign networks, for example those that originate from another subnet beyond the router.

We can now create a network that is independent of physical location and group users into logical workgroups. For instance, if a department has users in three different locations, they can now provide access to servers and printers as if they were all in the same building.

VLAN Benefits

As we have seen, there are several benefits to using VLANs. To summarize, VLAN benefits include:

Increased performance
Improved manageability
Simplification of software configurations
Increased security options
Increased performance

Switched networks by nature will increase performance over shared devices in use today by reducing collisions. Grouping users into logical networks will also increase performance by limiting broadcast traffic to users performing similar functions within workgroups. Additionally, less traffic will need to be routed, and the latency added to routers will be reduced.

Improved manageability

VLANs provide an easy, flexible, less costly way to modify logical groups in changing environments. VLANs make large networks more manageable by allowing centralized configuration of devices located in assorted locations.

Simplification of software configurations

VLANs will allow LAN administrators to "fine tune" their networks by grouping users. Software configurations can be made the same across machines with the consolidation of a department's resources into a single subnet. IP addresses and subnet masks will be more consistent across the entire VLAN. These services can be more effectively deployed when they can span buildings within a VLAN.

Increased security options

VLANs have the ability to provide additional security not available in a shared network environment. A switched network delivers packets only to the intended recipients and packets only to other members of the VLAN. This allows the network administrator to segment users requiring access to sensitive information into separate VLANs from the rest of the general users regardless of physical location.

VLAN Limitations

There are a few limitations to using VLANs:

Device limitations
Port constraints
Device limitations

The number of Ethernet addresses than can be supported by each device is 500. This is a distribution of about 20 devices per port on a 25 port switch. In an ideal network situation, there is one device per port, for example, a printer, a workstation, and voice IP phone will require 3 ports. If you wanted to have one VLAN assignment for each port, then the maximum VLANs will equal 25.

Port Constraints

If a hub or switch is connected to one port, every port on that hub must belong to the same VLAN. Hubs do not have the capability to provide VLANs to individual ports, and VLANs can not be extended beyond the device port even if a switch capable of supporting VLANs is attached.