I’ve been doing case reviews again, and I came across a case about Power Supply Redundancy in Cisco Catalyst 6500. The customer wanted to add another linecard (module) in their Catalyst 6500, but when the linecard was inserted, the Cat6500 showed a 'power denied' message. The Cat6500 is currently using 2 2500W power supplies in redundant mode, and the customer wanted to replace it with their spare 3000W power supplies. To minimize/prevent service interruption, the customer is asking us if he can inter-mix power supplies with different capacity. He's planning to change power supplies one at a time.
This should have been an easy case to answer. But then, it showed that even if I gained a lot of experience thru the years, I could still show some stupidity. And mind you, this one is a long-standing stupidity.
You see, the engineer who handled the case was quick and confident on his answer. He simply said yes that the Cat6500 would be able to support the redundancy of power supplies with unequal capacities. He even informed the customer that in a scenario where there are power supplies with unequal capacity, the Switch would operate using the limit of the lower capacity power supply to ensure complete power redundancy.
If I had this case, I would have given the same answer. And then, it hit us in our face when the customer replied. Apparently, the customer took our word for it and tried swapping one of the power supplies. We’re correct on the part that it will be just fine to inter-mix power supplies of unequal capacity. It’s on the second part that we were proven wrong when the customer showed us a ‘show power’ command output.
The command output showed that the Cat6500 Switch assumed the limit of the higher capacity. When I saw this part from the case, I got surprised and my first thought is that this could be a bug! I have known/accepted it for a long time that in power redundancy, a Switch will assume the lower limit. It’s what I learned when I was first taught about power redundancy in Cat6500.
Being curious, I searched for the updated document for Power Supply Redundancy. After getting into the correct link, I then realized my stupidity. It specifically noted that for Cat6500 running IOS (Native), the power supplies with unequal capacities will both come up, and the Switch’s total power (wattage) is equal to the output of the power supply with the higher capacity.
| Power Supplies | Redundancy | Outcome |
| Equal Power Output (Wattage) | Enabled | Since both will have equal power drawn, one power supply can support the whole chassis load, when the other power supply fails. Also, load sharing will be in effect wherein each power supply will provide half of the total power requirement of the chassis. |
| Unequal Power Output (Wattage) | Enabled | In CatOS (Hybrid), if the output difference between the power supplies is less than 10%, redundancy will still be enabled. But if the difference is greater than 10%, redundancy will be disabled wherein only the power supply with the higher output will be enabled. In Native IOS, both power supplies will be up, and the total power rating of the Switch will be equal to the output of the power supply with higher capacity. |
| Equal or Unequal Power Output (Wattage) | Disabled | The power available to the Switch will be equal to the combined power output of the power supplies, regardless if they have equal or unequal power outputs. |
Now, you may ask what will happen if the 'active' power supply with higher capacity fails. How will the redundant power supply (with lower capacity) support the Catalyst 6500 Switch? Well, if the current power requirement of the whole Switch (chassis, supervisor(s), fan tray, and linecards) is lower than the capacity of the remaining power supply, there won't be any problem. But if the remaining power supply's capacity is not enough, the switch will shutdown (power-deny) components in this order:
- Power over Ethernet (PoE) devices. The Switch (or the supervisor - being the intelligent component) will shutdown PoE devices from high-numbered ports to low-numbered ports on modules from highest-numbered slots to the lowest-numbered slots (descending order).
- Modules (Linecards). The Switch will shutdown modules from the highest-numbered slots to the lowest-numbered slots (descending order) until the total power requirement will be less than the capacity of the remaining power supply. Supervisor linecards (or supervisor engines) and Switch Fabric Modules (SFMs) will be exempted or bypassed.
