View source for IEEE 802.1Q

{{short description|IEEE networking standard supporting VLANs}}

'''IEEE 802.1Q''', often referred to as '''Dot1q''', is the [[computer network|networking]] standard that supports [[virtual local area network]]ing (VLANs) on an [[IEEE 802.3]] [[Ethernet]] network. The standard defines a system of '''VLAN tagging''' for [[Ethernet frame]]s and the accompanying procedures to be used by [[Network bridge|bridges]] and [[Network switch|switches]] in handling such frames. The standard also contains provisions for a [[quality-of-service]] 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.{{efn|VLAN membership is determined by the frame's port or the port/protocol combination, depending on whether port-based or port-and-protocol-based VLAN classification is being used.}}  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]] of the [[IEEE 802]] 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.<ref>[http://www.ieee802.org/1/pages/802.1Q-2014.html 802.1Q-2014 - Bridges and Bridged Networks]</ref>

==Frame format==
{{wide image|Ethernet 802.1Q Insert.svg|1328px|Insertion of 802.1Q tag in an Ethernet frame}}

802.1Q adds a 32-bit field between the source [[MAC address]] and the [[EtherType]] 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.<ref>Per IEEE 802.1Q Annex G.2.3 ''Minimum PDU size''</ref><ref>{{cite web |url=https://www.cisco.com/c/en/us/support/docs/lan-switching/8021q/17056-741-4.html |title=Inter-Switch Link and IEEE 802.1Q Frame Format |publisher=[[Cisco Systems]] |access-date=2019-09-26}}</ref> 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.<ref>IEEE 802.1Q-2011 clause 9.6</ref>

{| class="wikitable" width=400px
|+802.1Q tag format
|-
! width=50%|16 bits
! width=9.375%|3 bits
! width=3.125%|1 bit
! width=37.5%|12 bits
|-
| rowspan="2" align="center" | TPID
| colspan="3" align="center" | TCI
|-
| align="center" | PCP
| align="center" | DEI
| align="center" | VID
|-
|}

;Tag protocol identifier (TPID)
:A 16-bit field set to a value of 0x8100{{efn|The prefix ''0x'' indicates [[hexadecimal]] notation}} 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|class of service (CoS)]] and maps to the frame priority level. Different PCP values can be used to prioritize different classes of traffic.<ref>IEEE 802.1Q ''I.4 Traffic types and priority values''</ref>
:;Drop eligible indicator (DEI)
::A 1-bit field. (formerly CFI{{efn|This field was formerly designated ''Canonical Format Indicator (CFI)'' with a value of 0 indicating a MAC address in [[MAC address#Bit-reversed notation|canonical format]]. It is always set to zero for Ethernet. CFI was used for compatibility between Ethernet and [[Token Ring]] networks. If a frame received at an Ethernet port had a CFI set to 1, then that frame would not be bridged to an untagged port.<ref>IEEE 802.1Q-2005 clause 9.6</ref>}}) May be used separately or in conjunction with PCP to indicate frames eligible to be dropped in the presence of congestion.<ref>IEEE 802.1Q-2011 clause 6.9.3</ref>
:;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]]) 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]] 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.<ref>IEEE 802.1Q-2005, 9.6 VLAN Tag Control Information</ref>

For frames (other than 802.3 frames) using [[Subnetwork Access Protocol]] (SNAP) encapsulation with an [[organizationally unique identifier]] (OUI) field of 00-00-00 (so that the protocol ID field in the SNAP header is an EtherType as specified in {{IETF RFC|1042}}), the EtherType value in the SNAP header is set to 0x8100 and the aforementioned extra 4 bytes are appended after the SNAP header.<ref>IEEE 802.1Q-2011 clause 9.4 Tag Protocol Identifier (TPID) formats</ref>  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]] 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.<ref>{{citation |url=http://www.cisco.com/image/gif/paws/29805/175.pdf |title=Understanding Baby Giant/Jumbo Frames Support on Catalyst 4000/4500 with Supervisor III/IV |archive-url=https://web.archive.org/web/20150402195656/http://www.cisco.com/image/gif/paws/29805/175.pdf |archive-date=2015-04-02}}</ref>

===Double tagging===
[[IEEE 802.1ad]] introduced the concept of double tagging. Double tagging can be useful for [[Internet service provider]]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.
{{wide image|TCPIP 802.1ad DoubleTag.svg|1328px|Insertion of 802.1ad double tag in an Ethernet frame}}

==Other protocols==
IEEE 802.1Q defines the [[Multiple VLAN Registration Protocol]] (MVRP), an application of the [[Multiple Registration Protocol]], 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]].

==See also==
* [[Cisco Inter-Switch Link]] (ISL), an older Cisco proprietary VLAN management protocol
* [[Dynamic Trunking Protocol]] (DTP), a Cisco proprietary protocol to negotiate trunking between two VLAN-aware devices
* [[Time-Sensitive Networking]] (TSN), a suite of enhancements to 802.1Q for realtime and time-critical data streaming
* [[VLAN Trunking Protocol]] (VTP), a Cisco proprietary VLAN management protocol

==Notes==
{{notelist}}

==References==
{{reflist}}

==Sources==
* {{Citation |title=IEEE Std. 802.1Q-1998, Virtual Bridged Local Area Networks |doi=10.1109/IEEESTD.1999.89204 |isbn=0-7381-1537-1}}
* {{Citation |title=IEEE Std. 802.1Q-2003, Virtual Bridged Local Area Networks|doi=10.1109/IEEESTD.2003.94280 |isbn=0-7381-3663-8|year = 2003}}
* {{Citation |title=IEEE Std. 802.1Q-2005, Virtual Bridged Local Area Networks|doi=10.1109/IEEESTD.2006.216285 |isbn=0-7381-3662-X}}
* {{Citation |title=IEEE Std. 802.1Q-2011, Media Access Control (MAC) Bridges and Virtual Bridged Local Area Networks|doi=10.1109/IEEESTD.2011.6009146 |isbn=978-0-7381-6708-4 }}
* {{Citation |title=IEEE Std. 802.1BR-2012, Virtual Bridged Local Area Networks—Bridge Port Extension  |doi=10.1109/IEEESTD.2012.6239543 |isbn=978-0-7381-7281-1}}
* {{Citation |title=IEEE Std. 802.1Q-2014, Bridges and Bridged Networks|doi=10.1109/IEEESTD.2014.6991462|isbn=978-0-7381-9433-2}}
** {{Citation |title=IEEE Std. 802.1Q-2014/Cor 1-2015, Corrigendum 1: Technical and editorial corrections |doi=10.1109/IEEESTD.2016.7374647 |isbn=978-1-5044-0112-8}}
* {{cite web|url=https://www.cisco.com/c/en/us/support/docs/lan-switching/8021q/17056-741-4.html|title=Inter-Switch Link and IEEE 802.1Q Frame Format|publisher=[[Cisco Systems]]|access-date=2019-01-10}}

{{IEEE standards}}

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[[Category:IEEE 802]]
[[Category:Ethernet standards]]
[[Category:Link protocols]]