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How many stack cables would be needed if we want to stack two 3850 switches : We have one 3850 switch and planning to add one more. So my guess is only one Power & One data stack cable would be fine. Or please let me know if it is mandatory to loop the cable with two Power & two data stack cable?

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How many stack cables would be needed if we want to stack two 3850 switches : We have one 3850 switch and planning to add one more. So my guess is only one Power & One data stack cable would be fine. Or please let me know if it is mandatory to loop the cable with two Power & two data stack cable?

Minimum (and improperly), one stacking cable (data). However this is a bad idea as any issue with this cable will split your stack into standalone devices.

Minimum and properly, two stacking cables (data). Providing the loop is the proper (and documented) way to stack these switches. If one cable gets damaged or needs to be disconnected for some reason, the stack remains a stack.

The power stacking cables are purely optional, but add flexibility which can be useful and cost saving depending on the deployment. This allows the switches to "share" their power supplies with each other. Again, the loop is the correct way to utilize this feature.

Let's say that your power needs on each switch is about 400W. With the power stacking cables and a single 1100W power supply in each switch, you can have fully redundant power. Or you could skip the power stacking cables and put two smaller power supplies in each switch, which means you have four power cables to plug into outlets (if you can't use power strips --policy or regulation reasons-- this may exceed your available electrical outlets).

The actual implementation on power (and if you need the power stack cables) is going to highly depend on your situation and the potential future needs of your deployment. For example, if you know that there will be a VoIP rollout next year and you will need more power for PoE phones, you may want to leave the additional power supply slot available to add more power at that time.

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A stack should be connected in a loop, with a stack cable for each switch. With two switches, that would be two stack cables, three switches three stack cables, etc. that means Switch 1 connects to Switch 2 that connects to Switch 3, etc. (up to a total of nine switches in a data stack), the last switch then connects back to Switch 1. With larger stacks, you may need to buy one longer stack cable.

Connecting the stack in a loop prevents splitting the stack when a switch or link fails. Not using a loop will create two separate stacks where each thinks it is the original stack.

The power stack works the same way, but there is a limitation of four switches for the power stack, so that more than four switches could be in a data stack, but you would have more than one power stack in the data stack. You also need to take into account the power used for the power stack, and the power supplies in each switch. You could get into a situation where you do not have enough power to power the stack, or you could have enough to power the stack in a failure, but not enough power to reboot a switch or the stack (rebooting require more power than running at a steady state).

  • Thanks for prompt response. I was aware about multiple switches we need to make loop. But i had doubt for two switches. – shailendra harinkhede Sep 23 at 15:33
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    It doesn't matter how many switches in a stack (as long as it is more than one). You could end up with two separate stacks that each thinks it is the stack. That can happen with two switches if the link between them fails. Each switch would think it is what remains of the original switch stack. – Ron Maupin Sep 23 at 15:35

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