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IEEE 802.1Q

January 07, 2023

IEEE 802.1Q, often referred to as Dot1q, is the networking standard that supports virtual local area networking (VLANs) on an IEEE 802.3 Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying procedures to be used by bridges and 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.[a] 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. One of the notable revisions is 802.1Q-2014 which incorporated IEEE 802.1aq (Shortest Path Bridging) and much of the IEEE 802.1D standard.[1]

Frame format

 
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.[2][3] 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.[4]

802.1Q tag format
16 bits 3 bits 1 bit 12 bits
TPID TCI
PCP DEI VID
Tag protocol identifier (TPID)
A 16-bit field set to a value of 0x8100[b] 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.[5]
Drop eligible indicator (DEI)
A 1-bit field. (formerly CFI[c]) May be used separately or in conjunction with PCP to indicate frames eligible to be dropped in the presence of congestion.[7]
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.[8]

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 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.[9] 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.[10]

Double tagging

IEEE 802.1ad introduced the concept of double tagging. Double tagging can be useful for internet service providers (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.

 
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

Notes

  1. ^ 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.
  2. ^ The prefix 0x indicates hexadecimal notation
  3. ^ This field was formerly designated Canonical Format Indicator (CFI) with a value of 0 indicating a MAC address in 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.[6]

References

  1. ^ 802.1Q-2014 - Bridges and Bridged Networks
  2. ^ Per IEEE 802.1Q Annex G.2.3 Minimum PDU size
  3. ^ "Inter-Switch Link and IEEE 802.1Q Frame Format". Cisco Systems. Retrieved 2019-09-26.
  4. ^ IEEE 802.1Q-2011 clause 9.6
  5. ^ IEEE 802.1Q I.4 Traffic types and priority values
  6. ^ IEEE 802.1Q-2005 clause 9.6
  7. ^ IEEE 802.1Q-2011 clause 6.9.3
  8. ^ IEEE 802.1Q-2005, 9.6 VLAN Tag Control Information
  9. ^ IEEE 802.1Q-2011 clause 9.4 Tag Protocol Identifier (TPID) formats
  10. ^ (PDF), archived from the original (PDF) on 2015-04-02

Sources

  • IEEE Std. 802.1Q-1998, Virtual Bridged Local Area Networks, doi:10.1109/IEEESTD.1999.89204, ISBN 0-7381-1537-1
  • IEEE Std. 802.1Q-2003, Virtual Bridged Local Area Networks, 2003, doi:10.1109/IEEESTD.2003.94280, ISBN 0-7381-3663-8
  • IEEE Std. 802.1Q-2005, Virtual Bridged Local Area Networks, doi:10.1109/IEEESTD.2006.216285, ISBN 0-7381-3662-X
  • 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
  • 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
  • IEEE Std. 802.1Q-2014, Bridges and Bridged Networks, doi:10.1109/IEEESTD.2014.6991462, ISBN 978-0-7381-9433-2
    • 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
  • "Inter-Switch Link and IEEE 802.1Q Frame Format". Cisco Systems. Retrieved 2019-01-10.

January 07, 2023
ieee, often, referred, dot1q, networking, standard, that, supports, virtual, local, area, networking, vlans, ieee, ethernet, network, standard, defines, system, vlan, tagging, ethernet, frames, accompanying, procedures, used, bridges, switches, handling, such,. IEEE 802 1Q often referred to as Dot1q is the networking standard that supports virtual local area networking VLANs on an IEEE 802 3 Ethernet network The standard defines a system of VLAN tagging for Ethernet frames and the accompanying procedures to be used by bridges and 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 a 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 One of the notable revisions is 802 1Q 2014 which incorporated IEEE 802 1aq Shortest Path Bridging and much of the IEEE 802 1D standard 1 Contents 1 Frame format 1 1 Double tagging 2 Other protocols 3 See also 4 Notes 5 References 6 SourcesFrame format Edit 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 2 3 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 4 802 1Q tag format 16 bits 3 bits 1 bit 12 bitsTPID TCIPCP DEI VIDTag protocol identifier TPID A 16 bit field set to a value of 0x8100 b 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 5 Drop eligible indicator DEI A 1 bit field formerly CFI c May be used separately or in conjunction with PCP to indicate frames eligible to be dropped in the presence of congestion 7 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 8 dd 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 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 9 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 10 Double tagging Edit IEEE 802 1ad introduced the concept of double tagging Double tagging can be useful for internet service providers 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 Insertion of 802 1ad double tag in an Ethernet frameOther protocols EditIEEE 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 EditCisco 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 VLAN Trunking Protocol VTP a Cisco proprietary VLAN management protocolNotes Edit 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 The prefix 0x indicates hexadecimal notation This field was formerly designated Canonical Format Indicator CFI with a value of 0 indicating a MAC address in 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 6 References Edit 802 1Q 2014 Bridges and Bridged Networks Per IEEE 802 1Q Annex G 2 3 Minimum PDU size Inter Switch Link and IEEE 802 1Q Frame Format Cisco Systems Retrieved 2019 09 26 IEEE 802 1Q 2011 clause 9 6 IEEE 802 1Q I 4 Traffic types and priority values IEEE 802 1Q 2005 clause 9 6 IEEE 802 1Q 2011 clause 6 9 3 IEEE 802 1Q 2005 9 6 VLAN Tag Control Information IEEE 802 1Q 2011 clause 9 4 Tag Protocol Identifier TPID formats Understanding Baby Giant Jumbo Frames Support on Catalyst 4000 4500 with Supervisor III IV PDF archived from the original PDF on 2015 04 02Sources EditIEEE Std 802 1Q 1998 Virtual Bridged Local Area Networks doi 10 1109 IEEESTD 1999 89204 ISBN 0 7381 1537 1 IEEE Std 802 1Q 2003 Virtual Bridged Local Area Networks 2003 doi 10 1109 IEEESTD 2003 94280 ISBN 0 7381 3663 8 IEEE Std 802 1Q 2005 Virtual Bridged Local Area Networks doi 10 1109 IEEESTD 2006 216285 ISBN 0 7381 3662 X 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 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 IEEE Std 802 1Q 2014 Bridges and Bridged Networks doi 10 1109 IEEESTD 2014 6991462 ISBN 978 0 7381 9433 2 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 Inter Switch Link and IEEE 802 1Q Frame Format Cisco Systems Retrieved 2019 01 10 Retrieved from https en wikipedia org w index php title IEEE 802 1Q amp oldid 1128288082, wikipedia, wiki, book, books, library,

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