Response: Airflow is important – terminology is key – The Network Sherpa

Network Sherpa has been blogging up a total storm lately. Take this article on which is front or back when working out airflow and comparing three vendors approaches to how you choose the product:

What does front-to-back mean?

Using terminology like ‘front-to-back’ assumes a common understanding on where the front of the device is. I always thought the ‘front’ of a networking device was the bit with all the ports and blinkly-lights and stuff. Emm.. bad assumption.

Yep. Standards – you might think we have them. Cisco, of course, has the most people interfering with a simple design process. Combined with their habit of producing fifty SKUs for a product when five would be enough.

For the Cisco Nexus 3064 the ‘front’ of the device is the side with the fans, power supplies and management interfaces. The ‘back’ of the device as the side with the ports. Arrrggghh!! However, it does allow you to order the ‘default part code’ which has front to back airflow’ designed to be deployed in a TOR environment with it’s ports nearest the servers’ NICs.

Juniper, smaller team and more focussed but still ……

To tackle the front/rear ambiguity, Juniper uses the terms ‘port’ or ‘FRU’ side in the datasheet for the QFX 3500. For example a TOR would use ‘port-side exhaust’. Kudos to Juniper for ensuring there is no ‘default’ airflow assumed. It would be sweet if they would provide a master-SKU for each airflow option, rather than choosing it for every FRU type though.

Unsurprisingly, Arista shows that they can channel “common sense” instead of corporate marketing diffusion:

What about Arista?

I have to give credit where it’s due. Arista tackled this issue elegantly for it’s 7050 switch by explaining the options clearly. Like Juniper, they don’t assume any default airflow.

You are forced to make a choice between two master-SKU’s; DCS-7050S-64-F (front-to-back) or DCS-7050S-64-R (rear-to-front).

Arista also preserves the definition of ‘Front’ being port-side and ‘Rear’ being PSU-side.

Lastly, Arista uses color coded PSUs and fans using classic design principles. Red for hot (exhaust), blue for cold (intake). Then they show you a picture on their datasheet. Thank you Arista.

With pictures. Add Network Sherpa to your RSS feed. He’s got some great stuff going on.

Airflow is important – terminology is key – The Network Sherpa.

  • Fazal Majid

    Why even have 2 different SKUs? Won’t fans spin in the opposite direction if you reverse the voltage, something that could be done using 10 cents’ worth of diodes and transistors? Shouldn’t fan rotation direction be something you can configure from the device’s CLI or IPMI?

    • john harrington

      I believe the the limitation is in the shape of the impeller-blades in the fan. Changing the direction of fan rotation wouldn’t reverse the flow of air, just make it less efficient. I’m sure it’s possible to have a software-controlled servo to rotate the fan unit or the tilt the individual fan blades. That would be pretty cool indeed, but would be more expensive and might increase the likelihood of part-failure.

  • john harrington

    Hey Greg, thanks for featuring my post, much obliged.

    • Etherealmind

      No problems. Like your work and look forward to meeting you at #NFD4

  • caskings

    I came close to being bitten by Cisco’s odd definition of what is front/back on their N2Ks. Thankfully I realised my error during while auditing my planned BOM. That could have been a very embarrassing and costly mistake on my part when it came time to build the new dc.

  • Hagen

    What’s particularly annoying about the SKU approach, is that there is another way. You don’t have to decide when you order. Check out the Cisco Catalyst 2360. Just pull the fan module, rotate, and re-insert.

    • Etherealmind

      Smaller switches have less thermal problems because they don’t generate a lot of heat and have lots of space inside.

      Big switches have highly developed and carefully managed airflows so the reversible concept doesn’t work as easily as you might think.

      • Hagen

        Certainly, no argument. N7Ks, and the like, are a very different physical beast compared with ToR switching. I suggested the 2360 as an example, simply because much of the discussion seemed to regarding ToR. Most of which tend to be small switches…