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 refers to a family of IEEE standards about local area networks and metropolitan area networks. ... A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. ... // In computer networking, trunking describes using multiple network cables or ports in parallel to increase the link speed beyond the limits of any one single cable or port. ... Ethernet is a large, diverse family of frame-based computer networking technologies that operates at many speeds for local area networks (LANs). ...

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 router. See also Cisco's proprietary DTP, VTP and ISL for information on inter-switch and inter-VLAN communication. Lan can stand for several things: A local area network Lan (airline) formerly LanChile Lan Peru Län, a kind of administrative division used in Sweden Lan Mandragoran, a fictional character in the Wheel of Time fantasy series by Robert Jordan. ... A virtual LAN, commonly known as a VLAN, is a logically segmented network mapped over physical hardware. ... media access control is where people try to access a control to the media. ... Links ANSI/IEEE Std 802. ... The spanning tree network protocol provides a loop free topology for any bridged LAN. The Spanning Tree Protocol, which is also referred to as STP, is defined in the IEEE Standard 802. ... The network layer is level three of the seven level OSI model. ... Dynamic Trunking Protocol (DTP), is a proprietary protocol developed by Cisco Systems for the purpose of negotiating trunking on a link between two switches participating in a VLAN, and for negotiating the type of trunking encapsulation (802. ... VTP stands for VLAN Trunking Protocol, a protocol used for configuring and administering VLANs on Cisco network devices. ... Cisco Inter-Switch Link (ISL) is a Cisco Systems proprietary protocol that maintains VLAN information as traffic flows between switches and routers, or switches and switches. ...

As an illustration of the utility of VLANs, consider a company whose IT department wishes to provide separate logical networks for each department in the company while using only one physical corporate network. The IT department assigns a unique VLAN per department. Edge switches on the corporate network are configured to insert an appropriate VLAN tag into all data frames arriving from equipment in a given department. After the frames are switched through the corporate network, the VLAN tag is stripped before the frame is sent back to the department's equipment, possibly at a different geographical location. In this way, traffic from one department cannot be leaked to or snooped from another department.

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 Tag Protocol ID (TPID) hex 8100, identifying this frame as an 802.1Q frame. It then inserts an extra two-bytes of Tag Control Information (TCI) after the TPID, followed by another two bytes containing the frame's original EtherType. Together the four bytes of TPID and TCI are called the VLAN Tag. Ethernet v2 framing, also known as DIX Ethernet (named after the major participants in the framing of the protocol: Digital Equipment Corporation, Intel, Xerox) differs from IEEE 802. ... EtherType is a field in the Ethernet networking standard. ... EtherType is a field in the Ethernet networking standard. ...

The format of the TCI is

• user_priority: a 3-bit field storing the 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 bridged to an untagged port.
• VLAN ID (VID): a 12-bit field specifying the VLAN to which the frame belongs. A value of 0 means that the frame doesn't belong to any VLAN; in this case the 802.1Q tag specifies only a priority and is referred to as a priority tag. A value of hex FFF is reserved for implementation use. All other values may be used as VLAN identifiers, allowing up to 4094 VLANs. On bridges, VLAN 1 is often reserved for management.

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, the EtherType value in the SNAP header is set to hex 8100 and the aforementioned extra 4 bytes are appended after the SNAP header. IEEE 802. ... IEEE 802. ... The SubNetwork Access Protocol (SNAP) is a standard for the transmission of IP datagrams over IEEE 802 networks. ...

Because inserting this header changes the frame, 802.1Q encapsulation forces a recalculation of the original FCS field in the Ethernet trailer. It also increases the maximum frame size by 4 bytes. A frame check sequence (FCS) refers to the extra checksum characters added to a Frame in a communication protocol for error detection and correction. ...

Double-tagging can be useful for Internet Service Providers, allowing them to use VLANs internally while mixing traffic from clients that is already VLAN-tagged. The outer tag comes first, followed by the inner tag. In such cases, an alternate TPID such as hex 9100, or even 9200 or 9300, sometimes may be used for the outer tag; however this is being deprecated by 802.1ad, which specifies 88a8 for service-provider outer tags. Triple-tagging is also possible.

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". In telecommunications, multiplexing (also muxing or MUXing) is the combining of two or more information channels onto a common transmission medium using hardware called a multiplexer or (MUX). ...

For example, if an 802.1Q port has VLANs 2, 3 and 4 assigned to it with VLAN 2 being the Native VLAN, frames on this VLAN that egress (exit) 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, but every port on a network device can have a different native VLAN. On trunk links which are built between two networking devices to transport multiple VLAN traffic over that trunk link, the native VLANs on both sides of the trunk can have different VLAN-IDs - but still the packets which go over that link belong to the same subnet.

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. GVRP stands for GARP VLAN Registration Protocol. ... The Generic Attribute Registration Protocol was defined by the IEEE to provide a generic framework so bridges (or other devices) could register and de-register attribute values, such as VLAN identifiers and multicast group membership. ...

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 Multiple Spanning Tree network protocol and algorithm provides a loop free topology for any LAN or bridged network. ... IEEE 802. ...

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

• standard (must accept license agreement)
• ISL & 802.1q Frame Formats