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

Chapter 6. Source Routing

Until now, the only bridges we have considered have been those that operate using the transparent algorithms discussed in Chapter 2, "Transparent Bridges." Transparent bridges can be used on all types of LANs, including Ethernets, Token Rings, FDDIs, and others. In this chapter, we look at a completely different beast, called a source routing bridge, which is used exclusively on Token Rings and some FDDI networks.

While both types of devices may be called bridges, it is important to recognize that there is almost nothing in common between transparent and source routing bridge operation. The two technologies were developed independently by different groups of people from different companies. The technologies were also conceived from a completely different set of underlying assumptions, operate on completely different principles, and provide a completely different style of internetwork behavior. The world of bridged LANs is really divided in two: Token Rings that use source routing, and everything else. Source routing is not, and has never been, supported on Ethernet LANs. In section 6.6, we discuss methods for communicating between these parallel universes, except as noted in that section, the discussions of source routing in this chapter do not apply to Ethernet at all.

Following a brief overview of source routing operation, we will look at:

• The history of source routing

• The concepts underlying its design

• The details of source routing operation in end stations ...