Carroll, the co-owner of Indianapolis-based data center colocation company Lifeline Data Centers, said his company has large customers, some of which have a half-dozen or more Cisco network switches next to one another. The switches have side-to-side airflow, and that created problems.
"The way they had put the switches in the open-air racks, the exhaust air of one switch was blowing into the intake of another," he said. "It would just cascade on down the line, and three or four switches down the line, they were running really hot."
It is a common problem in data centers: How do you handle IT equipment that blows air in a different direction from your servers? Groups like the American Society of Heating, Refrigerating and Air-conditioning Engineers recommend hot-aisle/cold aisle design so that the backs of servers face one another in a row and, similarly, the front of servers face one another. But network switches can throw that setup into disarray.Why the side-to-side airflow?
Traditional network switch rack units have side-to-side airflow, mainly to squeeze in more line cards and therefore more ports. Why? As Doug Gourlay, senior director of data center solutions at Cisco, has said, with a switch that has side-to-side airflow, you can have the whole side of the chassis be the air inlet. With front-to-back configurations, up to half the vertically available real estate in the chasses is consumed by the air inlet.
"Front-to-back switches take up more vertical space and have a decreased port density," added Jim Fleming, a product design engineer at data center cabling company Panduit Corp.. "The number of line cards you can put in are limited by the width of the chassis," he said.
So, for example, one of the most popular network switches is the Cisco Catalyst 6500. It comes in both a side-to-side airflow chassis, and a front-to-back chassis. As of last year, only 10% of the switches sold were the front-to-back type.
Solving the network switch airflow problem
There are three principal ways to solve the data center network switch airflow problem: Buy a switch with a front-to-back chassis, turn the rack 90 degrees, and build or buy something to force the air backward.
Lifeline Data Centers chose the third option. The company has a sheet-metal shop on-site because it does a lot of data center construction. The company brought in its "sheet-metal guy," who then fabricated a metal duct in the shape of "half of a U," Carroll said. One end attached flush to the network gear, the other end pointed toward the hot aisle. So the hot air blowing out the exhaust of the network switches was diverted to the back instead of heading directly into the intake of the next switch.
If you don't have a sheet-metal worker on staff or the inclination to make it yourself, there are other options. Panduit sells ducts that can attach to the sides of network switches. Other companies like Chatsworth Products Inc. and Opengate Data Systems sell cabinets and enclosures that they say force air out the back.
Fleming of Panduit, suggested not designing the ducts yourself, because "it usually takes a couple iterations of [computational fluid dynamics] and real testing to come up with a viable solution for converting side-to-side to front-to-back."
"They're all back to normal temperature now," he said of his network switches.
Another option is to turn the rack 90 degrees, as Gourlay has suggested. That essentially turns a side-to-side airflow switch into a front-to-back switch. But there are problems with this method as well. Cabling management can become an issue, especially if you want to turn a cable-dense network switch around. Cables can get in the way.
Another potential problem is accessing switches during maintenance. Carroll said if they are turned sideways, there should be at least three feet between each rack so network staff can access the switches and, in particular, the RJ-45 blocks.
Finally, there are switches with front-to-back airflow. Cisco sells them, but they sacrifice port density. Other options include new Hewlett-Packard network switches that have front-to-back airflow for installation on hot-aisle/cold aisle-designed data centers.