Stack Power Query 3750 / 3850

[GRANT3779]

Hi All,

I'm in the process of putting together a few different 3850 stacks. Have gone over quite a bit regarding the setup and also on POE Priorities / PowerStacks etc.

I then decided to have a look about one of our production stacks (5 x 3750X) we have in place that was put in by a 3rd party a while back. I am seeing the following. Can you think of any reason why it would be setup like (2 separate  Power Stacks) this or have they forgotten to switch on the stack power using the 

stack-power switch x port x enable on some of the power ports? I can't think of any other reason it would be like this.

sh stack-power detail
Power Stack           Stack   Stack    Total   Rsvd    Alloc   Unused  Num  Num
Name                  Mode    Topolgy  Pwr(W)  Pwr(W)  Pwr(W)  Pwr(W)  SW   PS
--------------------  ------  -------  ------  ------  ------  ------  ---  ---
Powerstack-5          SP-PS   Ring     2480    1000    796     684     4    5
Powerstack-4          SP-PS   Stndaln  1430    520     332     578     1    2

Power stack name: Powerstack-5
    Stack mode: Power sharing
    Stack topology: Ring
    Switch 1:
        Power budget: 190
        Power allocated: 190
        Low port priority value: 21
        High port priority value: 12
        Switch priority value: 3
        Port 1 status: Shut
        Port 2 status: Connected
        Neighbor on port 1: 0000.0000.0000
        Neighbor on port 2: Switch 3 - b0fa.ebad.dc00

    Switch 3:
        Power budget: 190
        Power allocated: 190
        Low port priority value: 22
        High port priority value: 13
        Switch priority value: 4
        Port 1 status: Connected
        Port 2 status: Connected
        Neighbor on port 1: Switch 1 - b0fa.eb7c.c780
        Neighbor on port 2: Switch 5 - b0fa.eb7c.c280

    Switch 5:
        Power budget: 190
        Power allocated: 190
        Low port priority value: 20
        High port priority value: 11
        Switch priority value: 2
        Port 1 status: Connected
        Port 2 status: Connected
        Neighbor on port 1: Switch 2 - 0006.f6bb.e380
        Neighbor on port 2: Switch 3 - b0fa.ebad.dc00

    Switch 2:
        Power budget: 910
        Power allocated: 226
        Low port priority value: 19
        High port priority value: 10
        Switch priority value: 1
        Port 1 status: Connected
        Port 2 status: Not connected
        Neighbor on port 1: Switch 5 - b0fa.eb7c.c280
        Neighbor on port 2: 0000.0000.0000

Power stack name: Powerstack-4
    Stack mode: Power sharing
    Stack topology: Standalone
    Switch 4:
        Power budget: 910
        Power allocated: 332
        Low port priority value: 22
        High port priority value: 13
        Switch priority value: 4
        Port 1 status: Shut
        Port 2 status: Not connected
        Neighbor on port 1: 0000.0000.0000
        Neighbor on port 2: 0000.0000.0000

Thanks

[Joseph W. Doherty · Accepted solution]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

3750-X stacks, using power cables between member switches, are limited to four units.  How large is this stack?

[GRANT3779]

Hi Joseph,

There are 5 switches in the stack. Looking at the above then can I assume Switch 4 will be gone completely if it loses Power?

I've just read there are the same limitations with the 3850. If I'm creating a stack of 5 for example, is there a best practice design regarding the stack power? e.g 3 switches in one power stack and 2 in the other? Rather than have one stand alone switch as above.

Thoughts?

Thanks

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

"There are 5 switches in the stack. Looking at the above then can I assume Switch 4 will be gone completely if it loses Power?"

Well there's no power redundancy from StackPower for the standalone, but does the unit have dual PS, sufficient to carry the load of that switch?

"I've just read there are the same limitations with the 3850. If I'm creating a stack of 5 for example, is there a best practice design regarding the stack power? e.g 3 switches in one power stack and 2 in the other? Rather than have one stand alone switch as above."

I'm unsure there's a "best practice", per se.  If you ask Cisco, I suspect they would suggest using a RPS, in a star configuration.  If using StackPower, for some additional power redundancy, splitting StackPower, across multiple members, makes sense.

BTW, when using StackPower, at least on 3750-Xs, there an amperage limit that the cables support.  This means you need to carefully analyze your PS locations versus your power draw needs, especially if a PS fails.  It also means, you may need to distribute some smaller wattage PSs through your PowerStack rather than having only a couple high wattage PSs.

[GRANT3779]

Yeah all the switches have dual PSU, which I always forget about.

Is there much benefit to using stack power if all switches in a stack have 1:1 dual PSU?

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Well that depends on whether either PS can carry the switch's whole load.  If it can, your power redundancy is pretty well covered by the dual PSs.

In your case, you're dealing with an existing stack, but if you were designing a stack from scratch, you can save a few dollars by scaling back on PSs because you can cover some PS failures, or PoE deficiencies, with the StackPower.

[GRANT3779]

Thanks for the replies and insight as always Joseph.