IEEE 802.1Q

IEEE 802.1Q is the networking standard that supports Virtual LANs (VLANs) on an 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 provides provisions for a quality of service prioritization scheme commonly known as IEEE 802.1p and defines the Generic Attribute Registration Protocol

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 revisions including IEEE 802.1ak, IEEE 802.1Qat and IEEE 802.1Qay.

Example application

A company wishes to provide data separation and security between network traffic from its various departments by creating separate logical networks for each of its departments dispersed throughout the enterprise, while using only one corporate physical network. A network administrator assigns a unique VLAN to each department. Edge switches on the corporate network are configured to insert an appropriate VLAN tag into all data frames arriving from equipment belonging to a given department. After the frames are transmitted on their respective VLANs through the corporate network, the VLAN tag is stripped before the frame is sent to another computer belonging to the same department.

Frame format

Insertion of 802.1Q Tag in an Ethernet frame

802.1Q does not actually encapsulate the original frame. Instead, for Ethernet frames, it adds a 32-bit field between the source MAC address and the EtherType/Length fields of the original frame. 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, CFI, and VID.

16 bits	3 bits	1 bit	12 bits
TPID	TCI
	PCP	CFI	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/Length field in untagged frames, and is thus used to distinguish the frame from untagged frames.

• Tag Control Identifier (TCI)
  • Priority Code Point (PCP): a 3-bit field which refers to the IEEE 802.1p priority. It indicates the frame priority level. Values are from 0 (best effort) to 7 (highest); 1 represents the lowest priority. These values can be used to prioritize different classes of traffic (voice, video, data, etc). See also Class of Service or CoS.
  • Canonical Format Indicator (CFI): a 1-bit field. If the value of this field is 1, the MAC address is in non-canonical format. If the value is 0, the MAC address is in canonical format. It 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 Identifier (VID): a 12-bit field specifying the VLAN to which the frame belongs. The hexadecimal values of 0x000 and 0xFFF are reserved. All other values may be used as VLAN identifiers, allowing up to 4094 VLANs. The reserved value 0x000 indicates that the frame does not belong to any VLAN; in this case, the 802.1Q tag specifies only a priority and is referred to as a priority tag. On bridges, VLAN 1 (the default VLAN ID) is often reserved for a management VLAN; this is vendor-specific.

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 0x8100 and the aforementioned extra 4 bytes are appended after the SNAP header.^{[citation needed]}

Because inserting the VLAN tag 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.

Double tagging

With the IEEE standard 802.1ad, double-tagging can be useful for Internet service providers, allowing them to use VLANs internally while mixing traffic from clients that are 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 DoubleTag in an Ethernet frame

Non-standard triple-tagging is also possible. The third tag of 4 bytes allows extended addressing and also a small hop-count. The 66-bit addressing plan now uses a fixed (non-stacking) QinQinQ format. The result is three 32-bit tags plus the 16-bit EtherType/Length for a total of 112 bits. The two 48-bit (MAC) address fields add another 96 bits. The total header is 208-bits compared to a 320-bit IPv6 header. The 66-bit addressing is 18+48. The 18-bits are encoded 6-bits per 32-bit tag in the 12-bit VID fields. The 16-bit EtherType/Length field can contain the Payload Size or an EtherType for Payloads that contain their own Length, such as IPv4.

16 bits	3 bits	1 bit	12 bits
TPID0	PCP	CFI	VID0
TPID1	CONTENT RATING	CFI	VID1
TPID2	HOP	CFI	VID2

The contents of TPID0+TPID1+TPID2 contain the 48-bit MAC Address of the Source Device.

Trunk ports and the native VLAN

Clause 9 of the 1998 802.1Q standard defines the encapsulation protocol used to multiplex VLANs over a single link, by adding VLAN tags. However, it is possible to send frames either tagged or untagged, so to help explain which frames will be sent with or without tags, some vendors (most notably Cisco) use the concepts of a) trunk ports and b) the native VLAN for that trunk.

The concept of a trunk port is that once a port is designated as a trunk port, it will forward and receive tagged frames.

Frames belonging to the native VLAN do NOT carry VLAN tags when sent over the trunk. Conversely, if an untagged frame is received on a trunk port, the frame is associated with the Native VLAN for this port.

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 VLAN 2 that egress (exit) the aforementioned port are not given an 802.1Q header (i.e. 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 as to be expected - frames arriving for VLANs 3 & 4 are expected to be carrying tags that identify them so, and frames leaving the port for VLANs 3 & 4 will carry their respective VLAN tag.

Note that in this case, frames received on the port and tagged with VLAN ID 2 shall still be assigned to VLAN 2, but since the VLAN configuration shall be symmetric between emitting and receiving bridges, the distant bridge may not process the returning frames : it shall expect a tagged VLAN 2 frame, but will receive only untagged frames for it, then either discard them or distribute them in the wrong VLAN (the one defined as the "untagged" one on his side).

Not all vendors use the concept of trunk ports and native VLANs. Annex D to the 1998 802.1Q standard uses the concept of trunk links, but the current (IEEE Std 802.1D- 2004) standard does not use the terms trunk or native. Some use the term "Qtrunk" to avoid confusion with 802.3ad "link aggregation" that is often named a trunk as well.

Multiple VLAN Registration Protocol

In addition, 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.

Multiple Spanning Tree Protocol

The 2003 revision of the standard included the Multiple Spanning Tree Protocol (MSTP) which was originally defined in IEEE 802.1s.

See also

• VLAN Trunking Protocol (VTP), a Cisco proprietary VLAN management protocol
• Cisco Inter-Switch Link (ISL), an older VLAN trunking protocol that is proprietary to Cisco
• Dynamic Trunking Protocol another Cisco proprietary networking protocol.

References

• IEEE Std. 802.1Q-2005, Virtual Bridged Local Area Networks. ISBN 0-7381-3662-X. http://standards.ieee.org/getieee802/download/802.1Q-2005.pdf. 
• ISL & 802.1q Frame Formats

External links