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3750X Switchstack Power Questions

Greetings,

I have a couple questions I'd appreciate a little guidance with.

I've been assigned a task to expand a 3750X switchstack consisting of 2 members (both WS-C3750X-48T-S) by adding an additional member of the same model.  The new switch will not be in the same rack as the existing stack, and so longer stackwise cables have been provided to allow the data stack in this configuration.  No worries.

My questions revolve around the power situation.  Both of the existing members have dual 350W power supplies, and stack-power cables between them.  The stack-power mode has been set to power sharing strict, as follows:

swkSANo>sh stack-
Power stack name: Powerstack-1
    Stack mode: Power sharing strict
    Stack topology: Ring
    Switch 1:
        Power budget: 223
        Low port priority value: 21
        High port priority value: 12
        Switch priority value: 3
        Port 1 status: Connected
        Port 2 status: Not connected
        Neighbor on port 1: e8b7.4851.6280
        Neighbor on port 2: 0000.0000.0000

    Switch 2:
        Power budget: 223
        Low port priority value: 22
        High port priority value: 13
        Switch priority value: 4
        Port 1 status: Connected
        Port 2 status: Not connected
        Neighbor on port 1: e8b7.4851.2d80
        Neighbor on port 2: 0000.0000.0000


swkSANo>sh env power
SW  PID                 Serial#     Status           Sys Pwr  PoE Pwr  Watts
--  ------------------  ----------  ---------------  -------  -------  -----
1A  C3KX-PWR-350WAC     LIT15071C59  OK              Good     Good     350/0
1B  C3KX-PWR-350WAC     LIT14500ZXW  OK              Good     Good     350/0

swkSANo-2#sh env power

SW  PID                 Serial#     Status           Sys Pwr  PoE Pwr  Watts

--  ------------------  ----------  ---------------  -------  -------  -----

2A  C3KX-PWR-350WAC     LIT15071MKF  OK              Good     Good     350/0

2B  C3KX-PWR-350WAC     LIT14500ZY5  OK              Good     Good     350/0

I have 2 queries here. 

1 - Since the stack-power group is only 2 members, is it OK that only one stack-power cable is utilized, or should I attach the other cable? 

2 - Since there is no PoE, and the power budget is then presumably rather static, would I be better off changing the mode to redundant, or does powersharing accomplish the same thing in my scenario? 

Last question is related to the new stack-member.  The new member does not have redundant PS, just a single 350W PS.  I do not have longer stack-power cables to loop the new member in, so my thought is this:

Take one of the power supplies out of the existing stack and install it in the new stack member.  This way, any one of the 5 total PS can fail, and there will be (presumably) no impact to the operation of any member of the the stack.  Does my logic hold water here? 

Thanks in advance!

B.

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1 REPLY
VIP Super Bronze

3750X Switchstack Power Questions

Hi,

To have full redundancy, you want to connect the second power cable.  Have a look at table-9 in this link:

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps6406/white_paper_c11-578931.html

Regarding your second question, I think you should be fine with strict mode.  Default is power-sharing.

Power Share "Strict" Mode

The  power sharing mode has an inherent capability to oversubscribe itself  in case of power supply failures. That is, when an imbalance occurs  between available power budgets and allocated power, more power is  allocated than is available. By definition, the system should never have  a negative power budget, but it is possible that a power supply failure  brings the power budget below the allocated power. Such a condition  will trigger a series of alarming messages to the console.

HTH

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