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Tip of the Day
Language: Web Development
Expertise: Beginner
Apr 14, 1997

What are virtual LANs and how do they function?

Question:
What are virtual LANs and how do they function?

Answer:
Virtual LANs, as the name indicates, are software-defined groups of local area network nodes. These nodes are comprised of workstations, servers, printers, etc. Creating these virtual groupings of nodes offers many advantages, such as advanced security, greater control and ease of administration.

According to the ATM LAN Emulation specification, one of the requirements for using ATM as a medium for local area networks is that these LANs must be emulated (Local Area Network Emulation or LANE) or in other words, virtual.

In order to set up VLANs, we first start with a switched network. Typically, the switch vendor supplies the VLAN definition utility. This utility will help us define the individual virtual LANs and specify what network nodes are to be included in each VLAN. The factors involved in defining VLANs can be subnet addresses, switch ports, MAC (media access control) addresses of the nodes, protocol type, or even the type of application being used on the LAN.

Another way of analyzing the construction of VLANs is to use the OSI reference model. We can have two types of VLAN definitions, Layer 2 or the Data-Link Layer model, and Layer 3 or the Network Layer model.

Let's take a closer look at the data-link layer model. One of the easiest ways to create virtual LANs is to group the switch ports into virtual segments.

Note that if a hub were connected to a switch port on VLAN-1, then all devices connected to that hub would belong to VLAN-1. This is especially attractive when departmental VLANs are desired. Virtual segment VLANs are easy to design and administer and performance gains can be realized. The disadvantage is the fact that when a node has to be relocated to a different VLAN, a physical reconnection at the switch must take place.

Another Layer-2 method to create VLANs is the packet tagging principle. In this approach, when network nodes are designated as belonging to a specific VLAN, packets are tagged with unique additional information as they enter the switch. All switches will now examine this information prior to delivering the packet to its destination.

The third Layer-2 model relies on MAC addresses of network nodes. When VLANs are defined, tables are created for each VLAN and its member's MAC addresses are contained in these tables. Note that switching a user from one VLAN to another now does not require a physical reconnection, only the table entries need to be modified.

Layer 3 in the OSI reference model is also known as the Network Layer. Layer-3 VLAN segmentation techniques therefore employ network layer addresses. We can view these VLANS as subnets, hence the term virtual subnetting. Members of the same VLAN have the same subnet address. The switches use logic that reads the subnet address of the destination while forwarding packets. Note that all members of a particular VLAN will contain the same subnet address and therefore will be processed as members of the same bridge group. This translates to the following observation - all intra-VLAN traffic is bridged at Layer-2 while all inter-VLAN traffic is routed at Layer-3.

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