Virtual networks or virtual LANs (VLAN) are a technological concept for implementing logical workgroups within a network. Such a virtual network is implemented by means of LAN switching or by means of virtual routing on the link layer or on the network layer. Virtual networks are built by a set of switching hubs, which in turn are interconnected by a backbone. The IEEE802.1Qworking group has addressed this issue.
Classic network protocols are used in the backbone of virtual networks. Therefore, by design, there are no differences between the local area and a larger enterprise network consisting of several local areas, for example. Virtual networks use so-called "membership rules" to define the membership of stations to logical workgroups and implement switching networks to connect the members of the logical workgroups. This approach allows membership in a logical workgroup to be independent of the physical location of the workstation.
VLANs with LAN switching or virtual routing
Virtual networks form a broadcast domain based on physical LAN segments that can be freely defined according to certain criteria. Thus, there is a connection on the media access method( MAC) with or without additional evaluation of information from the network layer.
VLANs combine the advantages normally associated with networks connected by bridges, such as easy addition/removal or modification of a station, together with the advantage of logical system separation and structuring by means of routers, but without having to accept the throughput problems of bridges and the difficult configuration of large networks with routers. Virtual networks are implemented with switches of the type found in Dedicated Ethernet and with high-speed backbone technology.
One method of defining a virtual network is by assigning ports. All stations that are located at a certain port of an Ethernet switching system are considered to be part of the virtual network, and a set of Ethernet switch ports in the overall physical network form the entire virtual network. This can work transparently only on the condition that Ethernet switches are interconnected with scalable high-speed technology. A virtual network, if the right backbone technology is used, has few expansion limitations.
Packet switching on the virtual LAN
All traffic in the virtual network is realized with packet switching, i.e. addressing is done in the flat address space oflayer 2 addresses( data link addresses). However, the address space is limited to 4,096 VLAN IDs.
The switching systems have a learning algorithm similar to that of traditional bridges. This means that a station can easily change physical location and still remain a member of the virtual network without requiring reconfiguration at the end station. This allows for the creation of location-independent workgroups. In addition, data link address orientation has the advantage of protocol transparency: unlike router-based techniques, different layer 3 to 7 protocols can be used even within a workgroup. This design keeps traffic within a logical workgroup or within a virtual network. Broadcasts on one virtual network are in no way forwarded to another virtual network.
The virtual networks appear as completely independent switching fabrics. Therefore, the virtual networks shield their own traffic, which in turn makes routing between the virtual networks desirable, but without having to accept the coupling of logical workgroup and physical locations. However, this can basically be achieved in virtual networks by conventional routers. Because of the location transparency of the virtual network, one can place a router at a physical network location and connect it to as many physical ports as there are virtual networks. After that, one only has to assign each of these ports to a virtual network, and the router is accessible to all subscribers of this virtual network.