The Practical Guide to FDDI -- Bridging and Routing

As I said earlier, FDDI was around long before Fast Ethernet, and for many years it was really the only high-speed networking choice. It was never that common, since it was rather expensive, but it served a purpose. For example, supposing you've got a bunch of machines with Ethernet, and there's enough traffic that you're willing to invest in a switch, which was also rather expensive at that time. You've got a single connection to the backbone, and all the other machines access yours via that backbone. Wouldn't you like the backbone to be fast enough that each of your machines can use the full bandwith of its Ethernet connection?

Thus came to pass a breed of switches with multiple low-speed ports (Ethernet, Token Ring) and an FDDI connection. They're not that common, but they are out there, and if that sort of bottleneck between your FDDI ring and the rest of your LAN is acceptable, then you're set. Once again, look on Ebay.

If you want to bridge between FDDI and Fast Ethernet, you've got a much tougher row to hoe. It's easier to find routers which can do both, or to construct one using a machine for which you can get both types of cards and an OS which can do packet forwarding. Any of the free Unixes will do, probably most other types of Unix as well, and even NT should be able to do the trick if you're stuck in Windows-land. If it's acceptable for the FDDI machines to be on a different subnet from the rest of your LAN, that's probably the way to go. It may also be possible to get some of the free Unixes to do bridging instead of routing, but it is a complex issue due to the differing packet sizes between the two. I've seen quite a bit of debate about whether Ethernet/FDDI bridging is actually supported by anything yet, but no firm answers.

There are probably several models of switch which can do both FDDI and Fast Ethernet, but the only one that I'm familiar with is the HP Advancestack 2000. Even then, the FDDI cards for them are rare. You can still buy them from HP vendors... for only about $6000 each! I feel uniquely privileged to have two of them, and I'm using them, and you can't have them. :)

Joshua Boyd reports: The 3com LanPlex 2500 has two high-speed card slots and two low-speed card slots. The low-speed slots always seem to be used for 10base Ethernet, either 10baseT or 10baseFL. The two high speed slots have a few options. FDDI cards for that slot are extremely common. 100baseTX cards are fairly common (mine cost me $50 less than a year ago). And there are ATM cards which I've never actually seen in the wild. So, it might take some patience at this point, but you should be able to get a LanPlex 2500 with FDDI and 1 fast ethernet port for under $100.

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All text and pictures copyright (c) 2003 James Birdsall.