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  1. #1
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    Aug 2007
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    Best place to mount switches in 48U cabinet?

    I am going to be mounting (4) network switches in a 48U APC cabinet.
    I am trying to determine the best setup as to where to mount the switches.
    I have seen and heard different scenarios, top of rack with ports facing front of cabinet, top of rack with ports facing rear of cabinet, middle of rack, etc.
    I have used top of rack facing the front but did not like it as running the cables to the rear was a pain.

    I would prefer to have the ports facing the rear to make running cables to the servers in the rest of the rack much easier, and neater. But what about heat at the top of the rack, will the switch be taking in hot air with it facing the rear like that? I see pictures of it being done but it looks like it would be a potential heat issue for the switch.
    I will be installing a Cisco WS-C3750G-24TS-E, (2) HP Procure 2848, and one Cisco 2950G.

    I appreciate your suggestions for the setup you have found to work best and why.

  2. #2
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    Most of the Cisco switches draw their air from the side, so front or back mounting them most of the time doesn't really make a massive difference to the switches temp.

    However pay attention to where the air is coming out the switch and how your surrounding servers etc are been cooled. For example if you back mount the switch in the middle of the rack, and the switch blows out the back, you could be pushing a load of warmer/hot air into your cold aisle. Not really great if you have a load of expensive servers above the switch

    I would go for mounting the switch in the back of the rack at either the middle or top, I doubt you will have any issues.
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  3. #3
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    Most of the racks i have seen in my data center have the switches up top with the cable end facing the rear. They seem to mount at the very top and usually have a gap of 2U between them and the servers below them. Ive spoken to a couple techs in their from time to time when stoping in to do upgrades or check on my own equipment and they have never mentioned a switch failing or having problems with them failing...

  4. #4
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    Middle of the rack facing the rear... Allows for shorter cable lengths

  5. #5
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    Quote Originally Posted by SC-Daniel View Post
    Middle of the rack facing the rear... Allows for shorter cable lengths
    Yeh that's what I normally do, can look messy though without some cable management

    Although I suppose that's probably true for any arrangement.
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  6. #6
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    Quote Originally Posted by iTom View Post
    Yeh that's what I normally do, can look messy though without some cable management

    Although I suppose that's probably true for any arrangement.
    Yeah, anything without cable management is a mess

    I just setup a rack on Friday and the first thing I did was mount the switches in the middle rear.

  7. #7
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    Quote Originally Posted by SC-Daniel View Post
    Middle of the rack facing the rear... Allows for shorter cable lengths
    That's how I have been doing it lately, works great
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  8. #8
    top of rack facing rear was the easiest and most appropriate for my most recent deployment. as others have said, most of these switches draw air in from the sides, so it won't make a huge difference if you mount them front or rear. That said, if you do mount them rear, and the switches aren't too deep, that leaves room for other half depth equipment in the front of the rack, such as a pdu or whatever.

    The top also works well, because the top generally has some "dead" space where you can't mount anything anyway, and this gives you a good place to run cables. If you mount it lower in the rack, you may find yourself having to set aside valuable rack space to give yourself room to run all the ethernet cables. This goes doubly true for having a switch in the front of the rack.

    As to heat... the only time you should have an issue is if the rack / facility is not cooled properly in the first place, in which case you're going to be seeing hard drive failures before you see a switch failure. This was a big concern of mine initially, but it turned out not to really matter so long as the facility cooling was adequate in general.
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  9. #9
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    Quote Originally Posted by funkywizard View Post
    As to heat... the only time you should have an issue is if the rack / facility is not cooled properly in the first place, in which case you're going to be seeing hard drive failures before you see a switch failure. This was a big concern of mine initially, but it turned out not to really matter so long as the facility cooling was adequate in general.

    Google ran a study for a year on temperatures and drive failures, made absolutely no difference.

    http://labs.google.com/papers/disk_failures.pdf



    I've done it like this: http://hostthenpost.org/uploads/a82e...66260e3cd0.jpg

    And This: http://hostthenpost.org/tyler/2011-03-04_1842.png

    I like the idea of Middle mounting actually, may give that a shot.
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  10. #10
    Quote Originally Posted by Visbits View Post
    Google ran a study for a year on temperatures and drive failures, made absolutely no difference.

    http://labs.google.com/papers/disk_failures.pdf
    Good for them I still see heat related issues as a strong factor in hard drive failures. Google seem to have a rather tight control on their environmental conditions and their monitoring / reporting. The hard disk temperature graph never went above 50C in their test (120f). In not-so-ideal cases, say a fan fails in a server, and the hard drives run hotter than normal. All well and good if the rack is cooled adequately, but if it's not, then any little thing can push the drives to a temperature well beyond what it can reliably operate at. If the datacenter is having a bad day, and you have a fan that's flakey in the first place, or some other airflow problem, entirely possible that at least some of your drives are operating above the range in google's chart.

    In any case, my point was, in one case where I had a situation like this occur, I had 3 hard drive failures in one day due to extreme heat, at a previous place I hosted, but the switches didn't skip a beat. For a properly cooled datacenter, I wouldn't be too concerned with the switches overheating unless you did something stupid like block the fan grill.
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  11. #11
    Quote Originally Posted by Visbits View Post
    I've done it like this: http://hostthenpost.org/uploads/a82e...66260e3cd0.jpg

    And This: http://hostthenpost.org/tyler/2011-03-04_1842.png

    I like the idea of Middle mounting actually, may give that a shot.
    Both of those pics are set up in a way that will work fine for the most part, and I've done it like that before, but I didn't like the results that I got. You end up wasting U space to run the cables between or above / below the switches, and if you don't have a lot of free space between where the servers are mounted and the door on the cabinet, you have to really bend the cables to get the door closed. And then of course you need longer cables to make it all work. And then finally, it becomes more difficult to figure out which cable goes to which port, and remove the cables later on, if you need to. Obviously documentation is key there, but at the same time, you're better off not making your life any more difficult than it has to be. That configuration can certainly work, but I don't consider it ideal.
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  12. #12
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    Quote Originally Posted by funkywizard View Post
    Both of those pics are set up in a way that will work fine for the most part, and I've done it like that before, but I didn't like the results that I got. You end up wasting U space to run the cables between or above / below the switches, and if you don't have a lot of free space between where the servers are mounted and the door on the cabinet, you have to really bend the cables to get the door closed. And then of course you need longer cables to make it all work. And then finally, it becomes more difficult to figure out which cable goes to which port, and remove the cables later on, if you need to. Obviously documentation is key there, but at the same time, you're better off not making your life any more difficult than it has to be. That configuration can certainly work, but I don't consider it ideal.

    We can fit Copper GBICs into the 3550s and still close the doors without a problem, if we were to mount switches in back they would need to be at the very bottom or they would get cooked by vertically flowing heat.

    Switches run warm wherever you put them, this works good for us because we use 1U metered PDU that then branch off into unmetered strips on the right side, typically 6 strips per cab. (Lots of redundant psu on redundant power).

    Saving a couple U in a 48U cab is silly. If you need more space you need more space.


    The most important thing about building a rack is filling all empty space with fan trays or blanking plates, you do not want to leave space between servers open.

    We buy 10 packs of APC blanking planels all the time to fill empty areas.
    Last edited by Visbits; 03-13-2011 at 04:14 PM.
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  13. #13
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    For edge switches definitely in the middle of the rack (facing the back of the rack). Not only is it good for cable lengths but then the port numbers can easily be read/seen. When a switch is at the top of the rack you need a stool or a ladder to properly see which ports are which (from the labels) unless you count or go by the grouping (if switches have groupings).

    The dist-switches that are in one rack in the row (which all he other racks in that row have up-links go to) are put in the top of the racks (also facing backwards).

  14. #14
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    Sep 2010
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    I have always racked my (non-core) switches mid cabinet, with ports facing the hot aisle. However, all of my switches breath out the sides. If I had any that breathed out the rear of the switch, I'd likely mount them facing out the hot aisle.

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