IEEE 802.1Q, often referred to as Dot1q, is the networking standard that supports virtual local area networking (VLANs) on an

IEEE 802.3 IEEE 802.3 is a working group and a collection of standards defining the physical layer and data link layer's media access control (MAC) of wired Ethernet. The standards are produced by the working group of the Institute of Electrical and Electro ...

Ethernet Ethernet ( ) is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...

network. The standard defines a system of VLAN tagging for

Ethernet frame In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a data unit on an Ethernet link transports an Ethernet frame as its paylo ...

s and the accompanying procedures to be used by

bridges A bridge is a structure built to span a physical obstacle (such as a body of water, valley, road, or railway) without blocking the path underneath. It is constructed for the purpose of providing passage over the obstacle, which is usually somet ...

and

switches In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...

in handling such frames. The standard also contains provisions for a

quality-of-service Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...

prioritization scheme commonly known as IEEE 802.1p and defines the Generic Attribute Registration Protocol. Portions of the network which are VLAN-aware (i.e., IEEE 802.1Q conformant) can include VLAN tags. When a frame enters the VLAN-aware portion of the network, a tag is added to represent the VLAN membership. Each frame must be distinguishable as being within exactly one VLAN. A frame in the VLAN-aware portion of the network that does not contain a VLAN tag is assumed to be flowing on the native VLAN. The standard was developed by IEEE 802.1, a

working group A working group is a group of experts working together to achieve specified goals. Such groups are domain-specific and focus on discussion or activity around a specific subject area. The term can sometimes refer to an interdisciplinary collab ...

of the

IEEE 802 IEEE 802 is a family of Institute of Electrical and Electronics Engineers (IEEE) standards for local area networks (LANs), personal area networks (PANs), and metropolitan area networks (MANs). The IEEE 802 LAN/MAN Standards Committee (LMSC) main ...

standards committee, and continues to be actively revised with notable amendments including IEEE 802.1ad, IEEE 802.1ak and IEEE 802.1s. The 802.1Q-2014 revision incorporated the IEEE 802.1D-2004 standard.

Frame format

802.1Q adds a 32-bit field between the source

MAC address A MAC address (short for medium access control address or media access control address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use i ...

and the

EtherType EtherType is a two- octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the payload of the frame and is used at the receiving end by the data link layer to determine how the payload is processed. The same ...

fields of the original frame. Under 802.1Q, the maximum frame size is extended from 1,518 bytes to 1,522 bytes. The minimum frame size remains 64 bytes, but a bridge may extend the minimum size frame from 64 to 68 bytes on transmission. This allows a tag to be popped without needing additional padding. Two bytes are used for the tag protocol identifier (TPID), the other two bytes for tag control information (TCI). The TCI field is further divided into PCP, DEI, and VID. ;Tag protocol identifier (TPID) :A 16-bit field set to a value of 0x8100 in order to identify the frame as an IEEE 802.1Q-tagged frame. This field is located at the same position as the EtherType field in untagged frames, and is thus used to distinguish the frame from untagged frames. ;Tag control information (TCI) :A 16-bit field containing the following sub-fields: :;Priority code point (PCP) ::A 3-bit field which refers to the IEEE 802.1p class of service (CoS) and maps to the frame priority level. Different PCP values can be used to prioritize different classes of traffic. :;Drop eligible indicator (DEI) ::A 1-bit field. (formerly CFI) May be used separately or in conjunction with PCP to indicate frames eligible to be dropped in the presence of congestion. :;VLAN identifier (VID) ::A 12-bit field specifying the VLAN to which the frame belongs. The values of 0 and 4095 (0x000 and 0xFFF in

hexadecimal Hexadecimal (also known as base-16 or simply hex) is a Numeral system#Positional systems in detail, positional numeral system that represents numbers using a radix (base) of sixteen. Unlike the decimal system representing numbers using ten symbo ...

) are reserved. All other values may be used as VLAN identifiers, allowing up to 4,094 VLANs. The reserved value 0x000 indicates that the frame does not carry a VLAN ID; in this case, the 802.1Q tag specifies only a priority (in PCP and DEI fields) and is referred to as a ''priority tag''. On bridges, VID 0x001 (the default VLAN ID) is often reserved for a

network management Network management is the process of administering and managing computer networks. Services provided by this discipline include fault analysis, performance management, provisioning of networks and maintaining quality of service. Network managem ...

VLAN; this is vendor-specific. The VID value 0xFFF is reserved for implementation use; it must not be configured or transmitted. 0xFFF can be used to indicate a wildcard match in management operations or filtering database entries. For frames (other than 802.3 frames) using

Subnetwork Access Protocol The Subnetwork Access Protocol (SNAP) is a mechanism for multiplexing, on networks using IEEE 802.2 LLC, more protocols than can be distinguished by the eight-bit 802.2 Service Access Point (SAP) fields. SNAP supports identifying protocols by Et ...

(SNAP) encapsulation with an

organizationally unique identifier An organizationally unique identifier (OUI) is a 24-bit number that uniquely identifies a vendor, manufacturer, or other organization. OUIs are purchased from the Institute of Electrical and Electronics Engineers (IEEE) Registration Authority ...

(OUI) field of 00-00-00 (so that the protocol ID field in the SNAP header is an EtherType as specified in ), the EtherType value in the SNAP header is set to 0x8100 and the aforementioned extra 4 bytes are appended after the SNAP header. In other words the VLAN tag follows the SNAP header. For 802.3 frames in LLC-SNAP format, the order is opposite; the VLAN tag is placed ''before'' the LLC-SNAP header. Because inserting the VLAN tag changes the frame, 802.1Q encapsulation forces a recalculation of the original

frame check sequence A frame check sequence (FCS) is an error-detecting code added to a frame in a communication protocol. Frames are used to send payload data from a source to a destination. Purpose All frames and the bits, bytes, and fields contained with ...

field in the Ethernet trailer. The IEEE 802.3ac standard increased the maximum Ethernet frame size from 1518 bytes to 1522 bytes to accommodate the four-byte VLAN tag. Some network devices that do not support the larger frame size will process these frames successfully but may report them as ''baby giant'' anomalies.

Double tagging

IEEE 802.1ad introduced the concept of double tagging. Double tagging can be useful for

Internet service provider An Internet service provider (ISP) is an organization that provides a myriad of services related to accessing, using, managing, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, no ...

s (ISPs), allowing them to use their VLANs internally while carrying traffic from clients that is already VLAN tagged. The outer (next to source MAC and representing ISP VLAN) S-TAG (service tag) comes first, followed by the inner C-TAG (customer tag). In such cases, 802.1ad specifies a TPID of 0x88a8 for service-provider outer S-TAG.

Other protocols

IEEE 802.1Q defines the

Multiple VLAN Registration Protocol Multiple Registration Protocol (MRP), which replaced Generic Attribute Registration Protocol (GARP), is a generic registration framework defined by the IEEE 802.1ak amendment to the IEEE 802.1Q standard. MRP allows bridges, switches or other simil ...

(MVRP), an application of the

Multiple Registration Protocol Multiple Registration Protocol (MRP), which replaced Generic Attribute Registration Protocol (GARP), is a generic registration framework defined by the IEEE 802.1ak amendment to the IEEE 802.1Q standard. MRP allows bridges, switches or other simi ...

, allowing bridges to negotiate the set of VLANs to be used over a specific link. MVRP replaced the slower GARP VLAN Registration Protocol (GVRP) in 2007 with the IEEE 802.1ak-2007 amendment. The 2003 revision of the standard was the first to include the Multiple Spanning Tree Protocol (MSTP) which was originally defined in IEEE 802.1s.

Notes

References

Sources