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, for Ethernet frames using Ethernet II framing, it sets the EtherType value in the Ethernet header to hex 8100, identifying this frame as an 802.1Q frame, and then it adds an extra 4 bytes after the Ethernet header, consisting of a two-byte Tag Control Information (TCI) field with:

• 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.
• Canonical format indicator (CFI): a 1-bit indicator that is always set to zero for Ethernet switches. CFI is used for compatibility between Ethernet and Token Ring networks. If a frame received at an Ethernet port has a CFI set to 1, then that frame should not be forwarded as it is to an untagged port.
• VLAN ID (VID): a 12-bit field specify the VLAN to which the frame belongs. A value of 0 means that the frame doesn't belong to any VLAN (so that the 802.1Q header specifies only a priority), a value of 1 is used with bridges, and a value of hex FFF is reserved for implementation use; all other values may be used as VLAN identifiers, allowing up to 4093 VLANs.

and then a two-byte field containing the frame's original Ethertype. For frames using IEEE 802.2/SNAP encapsulation with an OUI field of 00-00-00 (so that the protocol ID field in the SNAP header is an EtherType), as would be the case on LANs other than Ethernet, it sets the EtherType value in the SNAP header to hex 8100 and appends the aforementioned extra 4 bytes after the SNAP header.

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)
• ISL & 802.1q Frame Formats