The IEEE have now produced a standard for Virtual Bridged Local Area Networks which is defined in IEEE 802.1Q. It employs explicit tagging to permit vLANs to span multiple switches. This is achieved through the addition of a tag header (immediately after the address fields) into a standard LAN frame. (Note that IEEE 802.3 has been modified to increase the maximum frame size by 4 octets).
The Tag header comprises a tag protocol identifier (TPID) and tag control information (TCI). The TPID uniquely identifies that the frame contains an IEEE802.1Q tag which for Ethernet LANs is coded as 8100. The TCI contains a 3 bit user priority, a 1 bit canonical format indicator and a 12 bit vLAN identifier.
The user priority field is capable of representing eight priority levels (0-7) and was defined within IEEE802.1p. The canonical format indicator indicates whether an Ethernet packet contains an additional routing information field.
The VLAN identifier uniquely identifies the vLAN to which the frame belongs. When set to 0 it indicates that the frame contains only priority information, a value of 1 is the default vLAN identifier and the value FFF is reserved. All other values can be used to represent separate vLANs.
The use of tagged frames extends to end stations. Each switch must therefore know whether to transmit frames in tagged or un-tagged format on a given port. Tagged format will always be used between switches but the use of tagged frames on other ports will depend upon whether or not the attached device supports that frame format.