Fiber Distributed Data Interface (FDDI), like ATM, was once a good choice for LAN backbones because of higher available speeds. But, just as with ATM, it appears to have been supplanted by the advent of high-speed Ethernet technology. In general, I don't advise implementing new FDDI networks without a compelling and unique requirement. However, it is important to understand FDDI because many networks still contain FDDI elements. If a network designer wants to upgrade a LAN infrastructure that contains FDDI components, she needs to understand how it works. It is likely that legacy FDDI installations are in critical parts of the network.
FDDI is a fiber optic-based networking protocol that uses two counter-rotating rings. I discussed it briefly in the previous chapter and showed how its clever fault tolerance system works. Since the protocol allows data transmission at 100Mbps, it was once useful as a network backbone. It has also been used effectively to create server farms close to the network Core Level. However, Fast Ethernet and 100Mbps Token Ring have effectively killed FDDI, and Gigabit Ethernet has rendered it obsolete. We still sometimes see legacy FDDI equipment supporting server farms. There are also some older implementations of network disk arrays and server clusters using FDDI that are still in operation.
You can build an FDDI network in two ways. The simple method is to connect fiber patch cords directly between the various ring components. Each device connects to the next, all the way around the ring. It is also possible to use an FDDI switch and construct a star topology. In this latter case, the ring becomes naturally full-duplex and closely resembles a switched Token Ring construction. Of course, it is possible to combine the two approaches, using the central switch as a multiport FDDI bridge.
Perhaps the simplest way to execute a phased removal of an FDDI backbone is to first bridge the FDDI ring to a Gigabit Ethernet switch. Then the FDDI devices can be migrated to the Ethernet switch one at a time. This way, there is no need to readdress the devices. This is important particularly in the case of server farms because readdressing servers may require changing configurations on a potentially large number of client devices.