Unsnarl port traffic


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The first question to ask yourself when choosing a switch is if you need a modular switch that allows you to simply add ports by adding line cards or if a fixed-port, nonmodular switch will suffice.

Nonmodular switches are available from vendors like Brocade Communications Systems Inc., Cisco and QLogic Corp., and port counts typically range from eight to 64 ports. All vendors, including Cisco with its MDS 9124 Multilayer Fabric Switch, now support ports on demand, which enables you to buy a fraction of the total number of available ports and simply activate the remaining ports by purchasing a license when additional ports are needed.

If the port count of a single nonmodular switch isn't high enough, multiple nonmodular switches can be cascaded using single or multiple trunked inter-switch link (ISL) connections. QLogic, with its SANbox 5200 and 5600 switches, made cascading a more acceptable option by offering a 10Gb/sec interconnect between each switch, eliminating the performance and management issues of switches linked via standard ISL links.

Furthermore, QLogic's recently released SANbox 9000 series stackable chassis switch connects to QLogic's modular switches via a 10Gb/sec high-speed FC link, providing a smooth transition path from nonmodular switches to director-level switches. The SANbox 9000 series stackable chassis switch differs from traditional directors from Brocade, Cisco and McData Corp. (recently

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acquired by Brocade) by limiting the total number of ports to 128, offering only nonblocking 16-port 4Gb/sec blades and four-port 10Gb/sec blades.

It's easy to add ports to a modular, chassis-based switch: Simply add line cards. Both Cisco and Brocade offer line cards with two, three or even four times (Cisco) the number of ports that originally came with their switch or director. Higher port-count line cards have a lower price per port, but they also have a higher level of oversubscription that can result in a less-predictable storage performance. "If you deal with oversubscription, you have to be mindful where you use it and you may need monitoring to deal with it," says Mario Blandini, Brocade's director of product marketing. "The use of oversubscription definitely makes storage management more complex, but at the same time, it lowers cost," he says.

High port-count line cards and oversubscription can also make cable management more challenging, and some of the existing racks may not be able to deal with it. "Large density can become a big problem, and if your cable management system isn't able to deal with it, you may end up just using every other port," says Blandini.

This was first published in April 2007

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