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

Chapter 12. Virtual LANs: The IEEE Standard

In Chapter 11, "Virtual LANs: Applications and Concepts," we examined VLANs from the perspective of application usage and technical concepts. In this chapter, we look at the technical details of the IEEE 802.1Q standard.

IEEE 802.1Q-2005 is the official industry standard for virtual bridged networks (this is standards terminology for a catenet comprising VLAN-aware switches). The standard itself closely mimics the IEEE 802.1D-2004 standard from which it was derived, but does not replace that earlier document; IEEE 802.1D-2004 is still the controlling document for VLAN-unaware switches. Much of IEEE 802.1Q comprises extensions to the concepts and architecture provided for VLAN-unaware devices rather than reinventing the canons of switched catenets altogether. In some cases, no changes at all were made to the 802.1D standard in order to provide for VLAN-aware operation. Much of the material in IEEE 802.1Q is literally copied from IEEE 802.1D and augmented to provide the necessary support for VLAN functionality.

This should not be taken to imply that a VLAN-aware switch could be built by making some simple modifications to an existing VLAN-unaware design. Nothing could be further from the truth. Support of VLAN capability implies some fundamental changes to switch architecture, particularly with respect to the structure and operation of the lookup tables (called the filtering database in the standard) and the application of VLAN behavioral ...