Nexus 9508 Switch Power Redundancy Modes

[Ahmet Mustafa Mungan]

Hi everyone,

We bought a 9508 model Nexus switch. It has 4x3KW power supplies on it. Our setup needs about 4KW power budget. I was researching about the power redundancy modes and there is something I could not understand. Cisco says there are three power redundancy mode. Those are:

Combined (nonredundant)	This mode does not provide power redundancy. The available power is the total power capacity of all power supplies.
insrc-redundant (grid redundancy)	This mode provides grid redundancy when you connect half of the power supplies to one grid and the other half of the power supplies to the second grid. The available power is the amount of power available through a grid. To enable grid redundancy, you must connect the power supplies to the correct power grid slots. For example, on the Cisco Nexus 9508 switch, slots 1, 2, 3, and 4 are in grid A, and slots 5, 6, 7, and 8 are in grid B. To configure and operate in grid redundancy mode, you must connect half of your power supplies to the slots in grid A and the rest of your power supplies to the slots in grid B. For more information on power grid slot assignments for your power supplies, see the Site Preparation and Hardware Installation Guide for your specific Cisco Nexus 9000 Series platform.
ps-redundant (N+1 redundancy)	This mode provides an extra power supply if an active power supply goes down. One power supply of all the available power supplies is considered an extra power supply, and the total available power is the amount provided by the active power supply units.

 Since our setup only needs for 4KW power and we have 12KW total available, even if I use combined mode, I can see no difference from other modes for redundancy. For example, if two of power supplies disable, there will be still 6KW there and I will have applied redundancy operationally. So why are those modes exist? I think all of them supplies the same redundancy in my scenario.

Kind Regards,

Ahmet Mustafa Mungan

[plustgraaf]

Unless you have a specific reason to use some other mode, I would use the combined mode.  It is the most flexible.

[joshua.james]

How would you suggest installing 2, 4, or 6 power supplies? Obviously 8 would fill the slots. However it seems that there is a special designation or wiring on the mid-plane for slots 1-4 versus 5-8. Should they be balanced between sides or simply installed in order?

[Edward Clear]

[I realize this is an ancient thread, but it came up in my searching and deserves an answer.]

The setting is all about how you want to design your power redundancy.  The three settings change how the switch calculates its "safe" Total Power Capacity.  So the Combined setting lets the switch allocate up to the total of all the PSUs, thus not guaranteeing any power redundancy.  The PS-Redundancy subtracts one PSUs worth from the safe total power budget, allowing for a single failure.  The grid redundancy mode is where you can slit the inputs of your power supplies between two power feeds.  So if either grid goes down, you keep running happily.  In this case it will calculate the safe power to allocate as the smaller of low and high slot PSU groups.

In OP's case, his current load was small enough that all the options were open to him.

Note that grid redundancy could even be considered if you can split your power inputs between two different breakers, not just grids.  Not nearly as good as full grid, but it does cover the case of a faulty breaker or local wiring.  Having two different power grid feeds to your facility that is carried through down to the switch obviously provides the maximum.

Of course understanding how the power load can increase is also fundamental.  Besides adding line cards, consider adding optics or varying PoE loads.