virtual LAN, commonly known as a VLAN, is a group of hosts with a common set of requirements that communicate as if they were attached to the same broadcast domain, regardless of their physical location. A VLAN has the same attributes as a physical LAN, but it allows for end stations to be grouped together even if they are not located on the same network switch. Network reconfiguration can be done through software instead of physically relocating devicesVLANs are created to provide the segmentation services traditionally provided by routers in LAN configurations. VLANs address issues such as scalability, security, and network management. Routers in VLAN topologies provide broadcast filtering, security, address summarization, and traffic flow management. By definition, switches may not bridge IP traffic between VLANs as it would violate the integrity of the VLAN broadcast domain.The shift toward LAN switching as a replacement for local/departmental routers” and now even shared media devices (hubs) will only accelerate in the future. With the rapid decrease in Ethernet and Token Ring switch prices on a per-port basis, many more ambitious organizations are moving quickly toward networks featuring private port (single user/port) LAN switching architectures. Such a desktop switching architecture is ideally suited to VLAN implementation. To understand why private port LAN switching is so well suited to VLAN implementation, it is useful to review the evolution of segmentation and broadcast containment in the network over the past several years.

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http://en.wikipediawiki/Virtual_LAN
http://micrel_PDF/Ethernet/White%20Paper/vlans%20wp.pdf

The Virtual LAN Technology Report

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Presented by :
David Passmore and John Freeman

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Introduction
Virtual LANs (VLANs) have recently developed into an integral feature of switched LAN solutions from every major LAN equipment vendor. Although end-user enthusiasm for VLAN implementation has yet to take off, most organizations have begun to look for vendors that have a well-articulated VLAN strategy, as well as VLAN functionality built into products today. One of the reasons for the attention placed on VLAN functionality now is the rapid deployment of LAN switching that began in 1994/1995. The shift toward LAN switching as a replacement for local/departmental routers” and now even shared media devices (hubs)” will only accelerate in the future. With the rapid decrease in Ethernet and Token Ring switch prices on a per-port basis, many more ambitious organizations are moving quickly toward networks featuring private port (single user/port) LAN switching architectures. Such a desktop switching architecture is ideally suited to VLAN implementation. To understand why private port LAN switching is so well suited to VLAN implementation, it is useful to review the evolution of segmentation and broadcast containment in the network over the past several years. In the early 1990s, organizations began to replace two-port bridges with multiport, collapsed backbone routers in order to segment their networks at layer 3 and thus also contain broadcast traffic. In a network using only routers for segmentation, segments and broadcast domains correspond on a one-to-one basis. Each segment typically contained between 30 and 100 users. With the introduction of switching, organizations were able to divide the network into smaller, layer 2“defined segments, enabling increased bandwidth per segment. Routers could now focus on providing broadcast containment, and broadcast domains could now span multiple switched segments, easily supporting 500 or more users per broadcast domain. However, the continued deployment of switches, dividing the network into more and more segments (with fewer and fewer users per segment) does not reduce the need for broadcast containment. Using routers, broadcast domains typically remain in the 100 to 500 user range. VLANs represent an alternative solution to routers for broadcast containment, since VLANs allow switches to also contain broadcast traffic. With the implementation of switches in conjunction with VLANs, each network segment can contain as few as one user (approaching private port LAN switching), while broadcast domains can be as large as 1,000 users or perhaps even more. In addition, if implemented properly, VLANs can track workstation movements to new locations without requiring manual reconfiguration of IP addresses. Why haven™t more organizations deployed VLANs? For the vast majority of end-user organizations, switches have yet to be implemented on a large enough scale to necessitate VLANs. That situation will soon change. There are, however, other reasons for the lukewarm reception that VLANs have received from network users up to now: ¢ VLANs have been, and are still, proprietary, single-vendor solutions. As the networking industry has shown, proprietary solutions are anathema to the multivendor/open systems policies that have developed in the migration to local area networks and the client server model. ¢ Despite the frequently quoted numbers illuminating the hidden costs of networking, such as administration and moves/adds/ changes, customers realize that VLANs have their own administrative costs, both straightforward and hidden. ¢ Although many analysts have suggested that VLANs enhance the ability to deploy centralized servers, customers may look at enterprise-wide VLAN implementation and see difficulties in enabling full, high-performance access to centralized servers. This paper discusses these and other issues in greater detail, and attempts to determine the strategic implications that VLANs, present and future, pose for enterprise networks.

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