Cisco: Oversubscribed by Design

As Cisco Systems Inc. (Nasdaq: CSCO) wades deeper into the SAN market, some of its competitors have been waging a whispering campaign that the MDS 9509 Fibre Channel switch is oversubscribed -- by a ratio of 4 to 1 -- if it's fully loaded with the maximum possible 224 2-Gbit/s ports. When a networking device is oversubscribed, it doesn't provide enough bandwidth for each port to send data at the full rate.

Cisco's response? Of course it's oversubscribed: We built it that way on purpose.

In fact, not only does Cisco own up to the fact that its 32-port line card for the 9509 is oversubscribed -- though it says the ratio is actually more like 3.2 to 1 -- it says the card gives SAN designers more flexibility than competing products.

"The 32-port card is not a compromise in performance," says Paul Dul, product line manager for the MDS 9000 at Cisco. "We've done this purposely because we want to provide differentiated pricing. It's a waste of bandwidth to deploy a full 2-Gbit/s to a host."

For its chassis-based director, Cisco offers a 16-port module, which it says is not oversubscribed and is intended for array connectivity, and the 32-port card for host connectivity. The latter has eight groups of four ports each; each of those groups shares 2.5 Gbit/s of bandwidth to the switch's backplane. Cisco says this architecture reduces the cost per port, with the 32-port line card priced around 60 percent that of the 16-port blade per port.