IEEE 802.1p/Q: VLAN Tagging and CoS

The IEEE 802.1Q specification defines a standards-based mechanism for providing VLAN tagging and class of service (CoS) across Ethernet networks. This is accomplished through an additional 4-byte tag, which carries VLAN and frame prioritization information, inserted within the header of a Layer 2 Ethernet frame, as shown in Figure 1.

 
Figure 1: Ethernet frame with 802.1Q tag

The 802.1Q tag has a specific format, consisting of four fixed-length fields. Two of the four fields carry the frame prioritization and VLAN information. Figure 2 illustrates the format of the 802.1Q tag itself.

 
Figure 2: IEEE 802.1Q tag format

The following are the fields within the 802.1Q tag:

• Tag Protocol Identifier (TPID): 802.1Q tagged Ethernet frames are indicated by a value of 0x8100 within this 16-bit field.
• Priority Code Point (PCP): A 3-bit field that indicates the frame priority level.
• Canonical Format Indicator (CFI): A 1-bit field set to 0 for Ethernet. Used for compatibility between Ethernet and Token Ring.
• VLAN Identifier (VID): A 12-bit field that specifies the VLAN to which the frame belongs.

The IEEE 802.1Q standard allows up to 4096 VLANS. To ensure interoperability with devices that do not support 802.1Q, the concept of the native VLAN was also introduced. Frames belonging to the native VLAN are not modified by the sending device (that is, the 4-byte 802.1Q tag is not added to the frame) when they are sent over a trunk. Likewise, any frames received over a trunk that does not have an 802.1Q tag are put into the native VLAN of the receiving device.

The operation of the three bits of the Priority Code Point (PCP) field is defined within the IEEE 802.1p standard, which is an extension of 802.1Q. The IEEE 802.1p and 802.1Q standards, therefore, work together to provide expedited traffic capabilities at the Layer 2 frame level. The IEEE 802.1p standard establishes eight levels of priority, referred to as CoS values. Table 1 illustrates the mapping of the CoS value to the bit field.

Table 1: CoS Value to PCP Bit Field Mapping 

COS Value	Bit Field
CoS 7	111
CoS 6	110
CoS 5	101
CoS 4	100
CoS 3	011
CoS 2	010
CoS 1	001
CoS 0	000

Common practice is to map different classes of traffic into different CoS values as they are sent across VLAN trunks. For example, VoIP traffic might be sent with a CoS 5 value, TelePresence sent with a CoS 4 value, and normal data traffic sent with a default CoS 0 value. Separate queues within network infrastructure devices that send and receive the frames then implement prioritization of the traffic classes.