IEEE 802.1Q overcomes the restrictions on router-based networks

Standard is set for Layer 1 and Layer 2 VLan products in a bid to reduce the costs to business of upgrading.

Collisions have always been a thorn in the side of network bid to reduce the costs to business of upgrading. managers.

The problem is that when a collision occurs, it is transmitted across the whole Lan, taking up valuable resources. Routers help to avoid this by keeping the collisions within a particular domain, but they are not the quickest of technologies.

VLans (virtual local area networks) were developed in part to overcome this problem, as they allow a network manager to logically segment a Lan into different domains. This is done through switches, as opposed to routers, meaning that fewer routers are needed and speeds are not effected.

As well as the performance advantages that we gain from the implementation of VLans, they offer a greater degree of flexibility. For example, a sales team working on a project with marketing and accounting staff can work on the same VLan as their colleagues.

Cutting out the cost of change

Another advantage is that up to 70 per cent of costs associated with networks come from additions, moves and changes. New cabling, workstations, and the time taken to implement this results in a massive outlay. VLans can therefore help reduce costs.

The core technology is relatively simple. When a Lan bridge receives data from a workstation it will tag the data with an identifier indicating the VLan that the data came from. There are several ways in which this can be done.

The first and most obvious is the port-based type. A bridge with four ports, for example, could have ports 1, 3, and 4 in VLan 1 and port 2 in VLan 2. This operates at Layer 1, and is the simplest method, but doesn't allow users to relocate away from an assigned bridge.

The second method is identification by the MAC address of each individual workstation. This operates at Layer 2. While this might sound like the most ideal solution, unfortunately it is a huge logistical challenge to set up initially.

Another Layer 2 method of identification is by protocol, so IP could be in a different VLan than IPX.

The most complex method is through membership by IP subnet address. This operates at Layer 3 and offers a lot of flexibility. However, it can often take longer to forward packets using Layer 3 information than it would using MAC addresses.

As ever, it was deemed necessary to introduce standards for VLan products, and the IEEE 802.1Q is the result. This means that network managers who want to implement VLans are able to do so without having to purchase all the equipment needed from the same vendor. It is worth noting, however, that only Layer 1 and 2 VLans have been defined in the standard. Layer 3 VLans remain vendor-specific.