VLAN Basic Concepts Explained with Examples

This tutorial explains basic concepts of VLAN, VLAN Membership (Static & Dynamic) and VLAN Connections (Access link & Trunk link) in detail with VLAN examples. Learn what VLAN is and what advantages it provides in computer network step by step.

What is VLAN

VLAN is a logical grouping of networking devices. When we create VLAN, we actually break large broadcast domain in smaller broadcast domains. Consider VLAN as a subnet. Same as two different subnets cannot communicate with each other without router, different VLANs also requires router to communicate.

Advantage of VLAN

VLAN provides following advantages:-

• Solve broadcast problem
• Reduce the size of broadcast domains
• Allow us to add additional layer of security
• Make device management easier
• Allow us to implement the logical grouping of devices by function instead of location

Solve broadcast problem

When we connect devices into the switch ports, switch creates separate collision domain for each port and single broadcast domain for all ports. Switch forwards a broadcast frame from all possible ports. In a large network having hundreds of computers, it could create performance issue. Of course we could use routers to solve broadcast problem, but that would be costly solution since each broadcast domain requires its own port on router. Switch has a unique solution to broadcast issue known as VLAN. In practical environment we use VLAN to solve broadcast issue instead of router.

Each VLAN has a separate broadcast domain. Logically VLANs are also subnets. Each VLAN requires a unique network number known as VLAN ID. Devices with same VLAN ID are the members of same broadcast domain and receive all broadcasts. These broadcasts are filtered from all ports on a switch that aren’t members of the same VLAN.

Reduce the size of broadcast domains

VLAN increase the numbers of broadcast domain while reducing their size. For example we have a network of 100 devices. Without any VLAN implementation we have single broadcast domain that contain 100 devices. We create 2 VLANs and assign 50 devices in each VLAN. Now we have two broadcast domains with fifty devices in each. Thus more VLAN means more broadcast domain with less devices.

Allow us to add additional layer of security

VLANs enhance the network security. In a typical layer 2 network, all users can see all devices by default. Any user can see network broadcast and responds to it. Users can access any network resources located on that specific network. Users could join a workgroup by just attaching their system in existing switch. This could create real trouble on security platform. Properly configured VLANs gives us total control over each port and users. With VLANs, you can control the users from gaining unwanted access over the resources. We can put the group of users that need high level security into their own VLAN so that users outside from VLAN can’t communicate with them.

Make device management easier

Device management is easier with VLANs. Since VLANs are a logical approach, a device can be located anywhere in the switched network and still belong to the same broadcast domain. We can move a user from one switch to another switch in same network while keeping his original VLAN. For example our company has a five story building and a single layer two network. In this scenario, VLAN allows us to move the users from one floor to another floor while keeping his original VLAN ID. The only limitation we have is that device when moved, must still be connected to the same layer 2 network.

Allow us to implement the logical grouping of devices by function instead of location

VLANs allow us to group the users by their function instead of their geographic locations. Switches maintain the integrity of your VLANs. Users will see only what they are supposed to see regardless what their physical locations are.

VLAN Examples

To understand VLAN more clearly let's take an example.

• Our company has three offices.
• All offices are connected with back links.
• Company has three departments Development, Production and Administration.
• Development department has six computers.
• Production department has three computers.
• Administration department also has three computers.
• Each office has two PCs from development department and one from both production and administration department.
• Administration and production department have sensitive information and need to be separate from development department.

With default configuration, all computers share same broadcast domain. Development department can access the administration or production department resources.

With VLAN we could create logical boundaries over the physical network. Assume that we created three VLANs for our network and assigned them to the related computers.

• VLAN Admin for Administration department
• VLAN Dev for Development department
• VLAN Pro for Production department

Physically we changed nothing but logically we grouped devices according to their function. These groups [VLANs] need router to communicate with each other. Logically our network look likes following diagram.

With the help of VLAN, we have separated our single network in three small networks. These networks do not share broadcast with each other improving network performance. VLAN also enhances the security. Now Development department cannot access the Administration and Production department directly. Different VLAN can communicate only via Router where we can configure wild range of security options.

So far in this article we have explained VLAN, in following section we will explain VLAN terms in more details.

VLAN Membership

VLAN membership can be assigned to a device by one of two methods

1. Static
2. Dynamic

These methods decide how a switch will associate its ports with VLANs.

Static

Assigning VLANs statically is the most common and secure method. It is pretty easy to set up and supervise. In this method we manually assign VLAN to switch port. VLANs configured in this way are usually known as port-based VLANs.

Static method is the most secure method also. As any switch port that we have assigned a VLAN will keep this association always unless we manually change it. It works really well in a networking environment where any user movement within the network needs to be controlled.

Dynamic

In dynamic method, VLANs are assigned to port automatically depending on the connected device. In this method we have configure one switch from network as a server. Server contains device specific information like MAC address, IP address etc. This information is mapped with VLAN. Switch acting as server is known as VMPS (VLAN Membership Policy Server). Only high end switch can configured as VMPS. Low end switch works as client and retrieve VLAN information from VMPS.

Dynamic VLANs supports plug and play movability. For example if we move a PC from one port to another port, new switch port will automatically be configured to the VLAN which the user belongs. In static method we have to do this process manually.

VLAN Connections

During the configuration of VLAN on port, we need to know what type of connection it has.

Switch supports two types of VLAN connection

• Access link
• Trunk link

Access link

Access link connection is the connection where switch port is connected with a device that has a standardized Ethernet NIC. Standard NIC only understand IEEE 802.3 or Ethernet II frames. Access link connection can only be assigned with single VLAN. That means all devices connected to this port will be in same broadcast domain.

For example twenty users are connected to a hub, and we connect that hub with an access link port on switch, then all of these users belong to same VLAN. If we want to keep ten users in another VLAN, then we have to purchase another hub. We need to plug in those ten users in that hub and then connect it with another access link port on switch.

Trunk link

Trunk link connection is the connection where switch port is connected with a device that is capable to understand multiple VLANs. Usually trunk link connection is used to connect two switches or switch to router. Remember earlier in this article I said that VLAN can span anywhere in network, that is happen due to trunk link connection. Trunking allows us to send or receive VLAN information across the network. To support trunking, original Ethernet frame is modified to carry VLAN information.

Trunk Tagging

In trunking a separate logical connection is created for each VLAN instead of a single physical connection. In tagging switch adds the source port’s VLAN identifier to the frame so that other end device can understands what VLAN originated this frame. Based on this information destination switch can make intelligent forwarding decisions on not just the destination MAC address, but also the source VLAN identifier.

Since original Ethernet frame is modified to add information, standard NICs will not understand this information and will typically drop the frame. Therefore, we need to ensure that when we set up a trunk connection on a switch’s port, the device at the other end also supports the same trunking protocol and has it configured. If the device at the other end doesn’t understand these modified frames it will drop them. The modification of these frames, commonly called tagging. Tagging is done in hardware by application-specific integrated circuits (ASICs).

Switch supports two types of Ethernet trunking methods:

• ISL [ Inter Switch Link, Cisco’s proprietary protocol for Ethernet ]
• Dot1q [ IEEE’s 802.1Q, protocol for Ethernet]

That's all for this part. In next part of this article we will practically implement what we have learnt from this part on Cisco switches.