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

3.1. Bridging the LAN Gap

While all of the standard LAN technologies may share the same addressing mechanisms, no one would argue that they are all the same. Each uses a different method for managing access to the LAN, each has a different frame format, and each provides a different set of features and services. In this section, we look at how it is possible to transparently bridge among these LANs.

The discussion uses examples of bridges among Ethernet (IEEE 802.3), Token Ring (IEEE 802.5), and FDDI (ISO 9314) LANs, primarily because these constitute the vast bulk of the installed base today. No other LAN technology supported by transparent bridging has any significant market share.^[] In addition, our focus is on the use of transparent bridges, as discussed in Chapter 2. No consideration is given (in this chapter) to Source Routing as used in Token Ring and FDDI or to interconnections between Source Routed and Transparently Bridged catenets.

^[] At the time of the writing of the first edition of this book, FDDI still had an installed base, but was not commonly used for new network installations. As such, the impact of FDDI has only decreased over time, and any FDDI bridging issues are becoming of historical, rather than practical, interest. At the time of this writing, Token Ring is now following the same fate as FDDI as most new LAN implementations are using Ethernet.

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