3750X power needs for PoE

[Mitchell Theriot]

I am building a switch stack using 4 48 port 3750X switches that will also have the power stacked.  If I install a single 715W power supply in each switch will the stack support 802.3af accross all 48 ports on each switch?  My calculations are 48 ports x 15.4W which gives me almost 740W needed which is over hte 715W power supply.  I was reading somewhere were it mentioned that in a powerstack additional power can be drawn from the stack.  I know this will not help if you are trying to support power on all switches accross all ports but would it if say 2 of the 4 switches are needing to provide PoE accross all 48 ports?  If I say we can only use 24 ports per switch for PoE that drops the power need down to 370W which I believe should work.  Just trying to get a better understanding of PoE consumption.  Would the best solution be to just add a second power supply to each switch?          

[Gregory Snipes]

Yes, a 715w power supply in each switch will allow you to supply 15.4w (Class 3) POE across every port on the stack.

However it will not allow you any redundancy. If one of the power supplies dies, you will no longer be able to provide 15.4w to every port. This can easily be remedied by putting a single extra 715w power supply into one of the switches. It does not matter witch one as the stack power system will (as long as it is configured to do so) move the power from the extra power supply to whatever switch needs it.

[Mitchell Theriot]

Would my best bet for redundancy be to add a second power supply to each switch in the stack?

[Gregory Snipes]

Depends on your situation. A redundant power supply in every switch will certainly be the most redundant solution as 4 power supplies could die before you would loose the ability to provide 15.4w to every port. But at the same time, unless you need any class 4 (30w) POE on the switches, the extra power supplies are pretty much going to be going to waste until you have two power supply failures.

The power supplies have a five year warenty so if one goes you will just not be redundant untill a replacement arrives from Cisco, for the next five years at least. You could always buy one extra and not install it as well so that you would have a spare to maintain your redundancy in case of a failure.

[dynamitec1]

StackPower cable that comes with 48-port 3750X should be enough for power redundancy.  Also, note that 3750X 48-port comes with 1100W as opposed to 715W.

Agreed with Gregory Snipes.  Having two power supplies for each is going to be a waste.

We have multiple buildings with 3750X using StackWise and StackPower with each having a single power supply.  If the power supply fails, remove it and replace with known good one...without losing power on any member of the stack.

HTH

[Gregory Snipes]

The 3750X 48 port can be ordered with either a 715w power supply (WS-C3750X-48P-L) or an 1100w power supply (WS-C3750X-48PF-L).

The 1100w power supply provides class 4 POE on every port with a single power supply.

[Mitchell Theriot]

I don't feel they will be using class 4 POE devices so I should be safe not going with 1100w power supply.  My redundancy can be provided through the powerstack cables but I was not sure if adding a second power supply to each switch in the stack would provide additional power support for class 2/3 devices.  The question being what would I gain by adding a second power supply to each switch in the stack.

[Gregory Snipes]

If you only need class 3 the only reason to add extra power supplies is redundancy. As long as the switches are IP Base you have stack power capability, so you really only need one extra power supply per power stack for that.

[Joseph W. Doherty]

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Posting

Indeed StackPower can share power across the stack; much as chassis power supplies share power across the whole chassis.

When you're computing power requirements, yes you need to account for the individual draw from each PoE port (or allow for maximum from every PoE port) but you also need to account for power for the switch itself (roughly 200 to 300 watts).

Normally, you would size each standalone switch individually, but with StackPower you can calculate the aggregate for multiple units, and provision sufficient power for that.  Additionally, you can often provision for redundancy, for example provide enough power that if even if your largest power supply fails, you have sufficient excess power to cover the loss.

StackPower has limitations on how much power can be shared across the power cables.  Because of this you may need to balance power supplies throughout the stack.  For example instead of running a stack of four on 3 1100W power supplies, you might find 6 715W power supplies would be necessary.

Additional details about StackPower can be found in this whitepaper: http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps6406/white_paper_c11-578931.html