IEEE 802.1Q was a project in the IEEE 802 standards process to develop a mechanism to allow multiple bridged networks to transparently share the same physical network link without leakage of information between networks. IEEE 802.1Q is also the name of the standard issued by this process, and in common usage the name of the encapsulation protocol used to implement this mechanism over Ethernet networks.

IEEE 802.1Q defines the meaning of a virtual LAN or VLAN with respect to the specific conceptual model underpinning bridging at the MAC layer and to the IEEE 802.1D spanning tree protocol.

Encapsulation

802.1Q does not actually encapsulate the original frame, it adds an extra 4-byte header to the original Ethernet header. That additional header includes a field -12 bit long, 4096 VLAN IDs- to identify the VLAN number. Because the original header has been changed, 802.1Q encapsulation forces a recalculation of the original FCS field in the Ethernet trailer.

Native VLAN

Clause 9 of the standard defines the encapsulation protocol used to multiplex VLANs over a single link, and introduces the concept of native vlan. Packets belonging to native vlan are not encapsulated when sending the frames over the trunk.

Generic Attribute Registration Protocol

In addition, IEEE 802.1Q defines GVRP, an application of the Generic Attribute Registration Protocol , allowing bridges to negotiate the set of VLANs to be trunked over a specific link.

Multiple spanning-tree protocol

The 2003 revision of the standard also rolled in the multiple spanning-tree protocol (MSTP) originally defined in IEEE 802.1s .

The official title of the 802.1Q standard is IEEE Std. 802.1Q-2003, Virtual Bridged Local Area Networks; ISBN 0-7381-3662-X.

External links

• 802.1Q-2003 standard (must accept license agreement)