Power Stacking for 3750x - Both cables needed?

ds6123 · ‎06-08-2012

Suppose I have a 3 member stack of 3750x switches. Members a, b and c.

I have all 3 members stacked with all 3 *data* stacking cables in a daisy-chain topology. The 50 cm data stacking cable from switch a (on the very top) to switch c (on the very bottom) just barely reached.

Of course, that means the 30 cm *power* stacking cable didn't reach between a & c. Right now, the power stacking cable goes from a to b, and then another cable from b to c. Leaving me with a useless extra 30 cm power stacking cable.

My question is, can switch "a" receive power from switch "c" through switch "b"? Is this just a big electrical "bus"? What if the power supply in switch "b" is removed?

Just wondering if there is a need for me to get a longer power stacking cable (one of the 150 cm power stacking cables). Would there be any benefit? Is only a certain amount of power shareable through each power stacking cable?

I read the config guide here and the faq here and I didn't see a clear answer.

Thanks!

Reza Sharifi · ‎06-08-2012

It is pool of power combined among all switches:

The Cisco Catalyst 3750-X and 3560-X Series Switches are designed for power supplies that provide a single power voltage. This approach simplifies the power supply design and allows aggregation of power, from power supplies in a single switch and across switches in a power stack. Cisco StackPower technology creates a pool of power that shares a common load consisting of all the switches in the power stack. This capability to manage power as share resource is unique to a stack of switches that can operate as a single unit. Therefore StackPower technology is available only on the Catalyst 3750-X switches.

Note that all power available in the power stack is combined into one single large pool of power, and the stack becomes a large single load to the power pool (Figure 2).

Here is the doc:

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

HTH

ds6123 · ‎06-08-2012

Thanks for the article Reza.

In that article, I found this little gem:

"Any switch can only share up to 2000W of power to a neighbor, whether the power is to be consumed by the neighbor or to be passed down to another switch in the stack."

So in my scenario, that makes it sound like switch "a" can send up to 2000W of power to switch "b" for "b" to use itself or pass it on to its neighbor (switch "c").

Good enough for me.

Thanks!

Joseph W. Doherty · ‎04-02-2018

Yea, glad you found that. Often overlooked, the 3750-X power cables have amperage limitations. I.e. you often cannot power a whole stack using just a few large power supplies in a few stack units.

One you can do, you can often "cover" the lost of one stack member's power supply from other stack units.

Khizersaleem · ‎04-02-2018

Hi Reza Can you please tell me one thing Is it safe to plug the power stacking cables while the stack switches are power on? What is the recommended way? Your reply will be highly appreciated. Best Regards