Spirent: FC Switches Fail to Scale

Using a new test that can simulate large-scale Fibre Channel fabrics, testing equipment vendor Spirent Communications says it has found that none of the FC switches it's tested is able to support fabrics that scale up to the theoretical maximum number of switches in a single fabric provided for in the Fibre Channel specification.

Spirent's Storage Routing Tester, to be available later this month, is a software module that runs on the company's SmartBits test system. The software, which has a list price of $40,000, is able to simulate up to 238 Fibre Channel E-ports per SmartBits port, making it the first test equipment to be able to generate SAN switch traffic in addition to that of end devices.

According to the industry's Fibre Channel specification, FC switches should be able to support up to 239 switches in a single fabric -- but none of them can actually do it, says Brian Mason, product manager for storage solutions at Spirent.

"We've found major scaleability concerns with all of the NEMs [network equipment manufacturers]," Mason says.

After the original version of this article was published, however, Spirent backtracked. The company sent Byte and Switch a statement that said, in part: "Spirent has worked with many vendors' switches but not all vendors and not all their products. As a neutral party, we cannot make a public assertion that no vendor in the industry can scale. Of the vendors our test equipment has tested, some stop at lower numbers and some numbers go beyond 200."Sadly, Spirent wouldn't tell us which vendors was able to scale "beyond 200" and which stopped "at lower numbers."

In any event, Spirent's comments reveal that some of the industry's players may be stretching the truth a bit. Three Fibre Channel switch vendors -- Brocade Communications Systems Inc. (Nasdaq: BRCD), Cisco Systems Inc. (Nasdaq: CSCO), and QLogic Corp. (Nasdaq: QLGC) -- claim their equipment is able to handle the theoretical maximum of the FC spec.

Brocade says its SilkWorm 12000 can handle up to 239 switches in one fabric, and Cisco and QLogic make the same assertions for their switches, according to the July Byte and Switch Insider (see FC Market Gets Rattled).

Meanwhile, McData Corp. (Nasdaq: MCDTA) and CNT (Nasdaq: CMNT) appear to be a little more reticent. McData says its switches can scale to a maximum of 24 devices in a single fabric, while CNT says its FC/9000 supports up to 56 in one SAN.

Mason says the only way to feasibly test these kinds of scaleability claims is to use Spirent's Storage Routing Tester. "Instead of having to set up 20 or 30 switches and up to 200 end devices, you just have one device with one GUI [graphical user interface] to control everything," he says.In actual throughput testing, Mason adds, the SmartBits system would be configured using multiple modules. For example, a customer might set up a test emulating 15 to 20 switches on each of five modules.

Besides discovering that none of the Fibre Channel vendors' switches could support anything close to a 239-switch SAN, Spirent also found that certain products were simply awful at recovering from major fabric disruptions. In one test, Spirent simulated an outage that took down five switches -- and the rest of the switches in the fabric required 20 seconds to reconverge. That's a process that should normally be measured in milliseconds.

Unfortunately, Mason wouldn't tell us which vendor's switches had performed so horribly. We're pretty sure one of its competitors would be willing to share this information at some point... unless, of course, every Fibre Channel switch suffers from the same glitch.

Todd Spangler, US Editor, Byte and Switch