Ethernet 802.1Q/p CoS | Marking Tools

Native Ethernet does not support any fields that can be marked for QoS purposes. However, when trunked with the 802.1Q protocol, three bits of the 802.1Q tag are made available for setting User Priority, referred to as 802.1p. These bits are more commonly referred to as CoS bits. Three bits allow for eight binary combinations for marking, represented by values 0 to 7.

Figure 1 illustrates the Ethernet frame with an 802.1Q tag, which includes the following fields:

• PRI: The Priority field, which is a 3-bit field that refers to the IEEE 802.1p priority. It indicates the frame priority level from 0 (lowest) to 7 (highest), which prioritizes different classes of traffic (such as voice, video and data).
• CFI: The Canonical Format Indicator, which is a 1-bit field. CFI is used for compatibility between Ethernet and Token Ring networks. It is always set to zero for Ethernet switches (and 1 for Token Ring networks).
• VLAN ID: The VLAN Identifier, which is a 12-bit field specifying the VLAN to which the frame belongs.

 
Figure 1: Ethernet frame: 802.1Q/p CoS field

You can set CoS fields with Class-Based Marking by using the set cos command. To use Class-Based Marking to set the 802.1Q/p Class of Service field to 2, enter the following:

Router(config-cmap)# policy-map L2-MARKER
Router(config-pmap)# class MARK-COS-2
Router(config-pmap-c)# set cos 2