The switch has two power supplies per system, allowing the power load to be split between them. This accommodates the increased maximum power of 30 watts per port provided to a powered device to meet the PoE+ standard (802.3at). With PoE+, a 48-port system would need 1440 Watts to provide 30 Watts per powered device for the PoE ports. Systems with fewer powered devices might require only one power supply. In this case, the additional power supply can provide one-to-one redundancy for the active supply.

In addition, the stacking switch supports StackPower, which allows the power supplies to share the load across multiple systems in a stack. By connecting the switches with power stack cables, you can manage the power supplies of up to four stack members as a one large power supply that provides power to all switches and to the powered devices connected to switch ports. Since power supplies are most effective when running at 30 to 90% of their maximum load, taking some of the power supplies offline provides maximum power efficiency. Switches in a power stack must be members of the same switch (data) stack.

You can configure the power stack to run in one of two modes:

• In power-sharing mode (the default), all input power is available to be used for power loads. The total available power in all switches in the power stack (up to four) is treated as a single large power supply, with power available to all switches and to all powered devices connected to PoE ports. In this mode, the total available power is used for power budgeting decisions and no power is reserved to accommodate power-supply failures. If a power supply fails, powered devices and switches could be shut down (load shedding).

• In redundant mode, the power from the largest power supply in the system is subtracted from the power budget, which reduces the total available power, but provides backup power in case of a power-supply failure. Although there is less available power in the pool for switches and powered devices to draw from, the possibility of having to shut down switches or powered devices in case of a power failure or extreme power load is reduced.

In addition, you can configure the mode to run a strict power budget or a non-strict (relaxed) power budget. In both modes, power is denied when there is no more power available in the power budget.

• In strict mode, when a power supply fails and the available power drops below the budgeted power, the system balances the budget through load shedding of powered devices, even if the actual power being consumed is less than the available power.

• In non-strict mode, the power stack is allowed to run in an over-allocated state and is stable as long as the actual power does not exceed the available power. In this mode, a powered device drawing more than normal power could cause the power stack to start shedding loads. This is normally not a problem because most devices do not run at full power and the chances of multiple powered devices in the stack requiring maximum power at the same time is small.

You configure power modes at a power-stack level (that is, the mode is the same for all switches in the power stack).

You can also configure a switch connected in a power stack to not participate in the power stack by setting the switch to standalone power mode. This mode shuts down both stack power ports.

You can configure the priority of a switch or powered device to receive power. This priority determines the order in which devices are shut down in case of a power shortage. You can configure three priorities per system: the system (or switch) priority, the priority of the high-priority PoE ports on a switch, and the priority of the low-priority PoE ports on a switch.

You set port priority at the interface level for powered devices connected to a PoE port. By default, all ports are low priority.

You configure the priority values of each switch in the power stack and of all high and low priority ports on that switch to set the order in which switches and ports are shut down when power is lost and load shedding must occur. Priority values are from 1 to 27; switches and ports with highest values are shut down first.

Note	The 27 priorities are used to accommodate power stacks connected in a star configuration with the expandable power supply. In this configuration, there would be nine members (switches) per system with three priorities per switch.

On any switch, the switch priority must be lower than port priorities. and the high priority value must be set lower than the low priority value. We recommend that you configure different priority values for each switch and for its high priority ports and low priority ports. This limits the number of devices shut down at one time during a loss of power. If you try to configure the same priority value on different switches in a power stack, the configuration is allowed, but you receive a warning message.

The default priority ranges, if none are configured, are 1-9 for switches, 10-18 for high-priority ports, and 19-27 for low-priority ports.

Load shedding is the process of shutting down devices in case of power supply, cable, or system failures. For power stacks in power-sharing mode, there are two types of load-shedding: immediate and graceful.

• Immediate load shed occurs when a failure could cause the power stack to fail very quickly. For example, if the largest power supply in the power stack fails, this could cause the stack to immediately start shutting down powered devices.

• Graceful load-shedding can occur when a smaller power supply fails. Switches and powered devices are shut down in order of their configured priority, starting with devices with priority 27, until the power budget matches the input power.

Graceful load shedding is always enabled and immediate load shedding occurs only when necessary, so both can occur at the same time.

Note	Load shedding does not occur in redundant mode unless two or more power supplies fail, because the largest power supply is used as a backup power source.

Notes on load shedding:

• The method (immediate or graceful) is not user-configurable, but is based on the power budget.

• Immediate load shedding also occurs in the order of configured priority, but occurs very quickly to prevent hardware damage caused by loss of power.

• If a switch is shut down because of load shedding, the output of the show stack-power privileged EXEC command still includes the MAC address of the shut down switch as a neighbor switch, even though the switch is down. This command output shows the StackPower topology, even if there is not enough power to power up a switch.

1.    configure terminal


2.    stack-power stack power stack name


3.    mode {power-sharing | redundant} [strict]


4.    end


5.    show stack-power

Switch# configure terminal

Switch(config)# stack-power stack power 1

Switch(config-stackpower)# mode redundant

Switch(config-stackpower)# end

Switch# show stack-power

1.    configure terminal


2.    stack-power switch switch-number


3.    stack [power-stack-name]


4.    power-priority switch value


5.    power-priority high value


6.    power-priority low value


7.    end


8.    show stack-power

Switch# configure terminal

Switch(config)# stack-power switch 4

Switch(config-switch-stackpower)# stack power2

Switch(config-switch-stackpower)# power-priority switch 5

Switch(config-switch-stackpower)# power-priority high 12

Switch(config-switch-stackpower)# power-priority low 20

Switch(config-switch-stackpower)# end

Switch# show stack-power

1.    configure terminal


2.    interface interface-id


3.    power inline port priority {high | low}


4.    end


5.    show power inline priority

Switch# configure terminal

Switch(config)# interface gigabitethernet 1/0/1

Switch(config-if)# power inline port priority high

Switch(config-if)# end

Switch# show power inline priority