This chapter discusses the design of flat (nonhierarchical or Layer 2) networks, using devices such as bridges and switches. These devices are not, strictly speaking, internetworking devices but are commonly used in network design, mostly in campus-based local area networks (although they may support wide area interfaces also). We will focus on the following issues:

• Bridging mechanisms and applications

• Switch mechanisms and applications

• Virtual LANs (VLANS)

Previously, we discussed how to build local and campus area networks, and one of the key features introduced was the idea that LAN segments are constrained in both length and the number of stations that may be attached. Many of these LAN technologies also employ shared bandwidth schemes, where overall traffic levels are directly related to the number of nodes added. Regarding IEEE 802.3/Ethernet networks we also discussed the concept of the collision domain, which introduces further constraints on network growth.

Bridging was the first real tool for improving scalability in local area networks, and in the early 1980s bridging technology was (hard to believe now) at the cutting edge. Bridges were also used to build wide area networks, though thankfully these applications are now rare (for reasons we will discuss later). Bridges have been largely eclipsed nowadays by their faster, slicker offspring switches and general-purpose multiprotocol bridge-routers. Bridging is actually still very much alive but now working in disguise.