Mixing arbitrated loop and fabric
Rick Cook
Although Fibre Channel fabric topology offers advantages in performance and speed, the alternative arbitrated loop (FC-AL) topology is still common, both for reasons of cost and historical precedence. The ability to attach FC-AL loops to a Fibre Channel fabric switch is desirable to preserve existing investments in FC-AL equipment and also because an arbitrated loop can provide adequate performance for some sub-groups at lower cost.
One of the problems in attaching arbitrated loop devices to a fabric switch comes from the different ways they handle their addresses. As a result, many arbitrated loop devices, such as HBAs, cannot be assigned an address by the switch on login, and therefore cannot initiate communications across the fabric.
One solution to the problem is to keep the arbitrated loops but replace the HBAs on them with those that are fabric-aware. Another is to use a Fibre Channel switch that provides a way to communicate with arbitrated loops. For example Vixel offers what it calls "Stealth Mode" on its 8100 fabric switch. With this system, devices are not required to perform a fabric login to communicate with other switched devices.
A white paper titled "Arbitrated Loop Attachment For Fibre Channel Switches" is available from the Fibre Channel Industry Association (FCIA) web site.
Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.
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