VLAN tagging is a method to tag VLAN information within the Ethernet frames on MAC level. VLAN tagging supports existing 802.x MAC services, provides prioritization mechanisms, is limited to port-based VLANs, and supports any virtual network with a spanning tree.
When tagging is used, compatibility and interoperability with existing local area networks( LAN) is maintained. This requirement is not without problems because tagging expands the header for the VLAN information, but at the same time preserves the MAC frame. The IEEE 802.3as: Frame Extensionworking group is concerned with the extension of the Ethernet frame, and the 802.1Q, 802.1ad and 802.1ah working groups are concerned with VLAN tagging.
To ensure a functioning tagging procedure, an incoming frame must be analyzed and forwarded to the transit systems with the corresponding VLAN information. These systems must know at which position of the MAC frame the tag is located and be able to evaluate it. Since the traditional MAC frames have a different structure, knowledge of the tag location is crucial for evaluation.
In VLAN tagging, an additional frame is inserted into the Ethernet frame, between the source address( SA) data field and the type field. The VLAN tag field consists of the 16-bit long data field, the Tag Protocol Identifier( TPID), and the likewise 16-bit long data field Tag Control Information( TCI), which in turn consists of three data fields: the Priority Code Point( PCP) in which the priority of the frame is defined, the Canonical Format Indicator( CFI) for port control of Ethernet switches and the 12-bit long data field for the VLAN Identifier( VID).