The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

13.3. VLAN and Priority Tagging

In Chapter 11, "Virtual LANs: Applications and Concepts," and Chapter 12, "Virtual LANs: The IEEE Standard," I discussed the use of Virtual LAN (VLAN) tagging to explicitly indicate the VLAN association of a given frame. A VLAN tag contains both a VLAN identifier and a priority value indicating the user priority assigned to the frame. Through the use of tags, a device can signal user priority on LANs (such as Ethernet) that provide no native priority support. It is possible to perform this feat even if the device in question does not support VLANs. The special case of a tagged frame carrying a VLAN Identifier of all zeroes is not, strictly speaking, a VLAN tag. It is referred to as a priority tagged frame, and is depicted in Figure 13-6. VLAN identifier 0x000 is specifically reserved for this purpose.^[]

^[] While extensively discussed and permitted by the standard [IEEE98d], Priority tagging in this manner is not common. Because the complexity of implementing tagging for priority is not significantly less than that of implementing tagging for VLANs, most devices that insert tags do so in a VLAN context and use a non-zero value for the VLAN Identifier.

Figure 13.6. Priority-tagged Ethernet frame

While Figure 13-6 depicts a Priority-tagged Ethernet ...