IEEE 802.1Q

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 (i.e. trunking). 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 also 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. This protocol allows for individual VLANs to communicate with one another with the use of a layer-3 (network) router. See also Cisco's proprietary VTP and ISL for information on inter-switch and inter-VLAN communication.

Frame format

802.1Q does not actually encapsulate the original frame. Instead, it adds an extra 4-bytes in the original Ethernet header. First, an EtherType of 0x8100 is inserted after the source address indentifing this frame as a 802.1Q frame. This is followed by a 2 byte header that contains the following fields:

• user_priority: this 3-bit field can be used to store a priority level for the frame. Use of this field is defined in IEEE 802.1p.
• CFI: a 1-bit flag denoting whether MAC addresses in the frame are in canonical format. This is called the Canonical Format Indicator.
• VID: a 12-bit VLAN ID, allowing up to 4096 VLANs.

and then the original ethertype etc.

Because inserting this header changes the frame, 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 a native VLAN. Frames belonging to the native VLAN are not modified when sent over the trunk. Native VLANs are also referred to as "Management VLANs".

For example, if a 802.1Q port has VLANs 2, 3 and 4 assigned to it with VLAN 2 being the Native VLAN, frames on this VLAN which are to egress(out) the aforementioned port are not given an 802.1Q header (ie., they are plain ethernet frames). Frames which ingress(Enter) this port and have no 802.1Q header are put into VLAN 2. Behaviour of traffic relating to VLANs 3 & 4 is intuitive.

Any given 802.1Q port can only have one Native VLAN.

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)