Stack-power mode is used only on Catalyst switches in a power stack. With no XPS, a power stack operates in ring topology
with a maximum of four switches in the stack. When an XPS is in the power stack, you can connect up to nine switches in the
stack plus the XPS, providing power budgets to power stack members similar to stack-power ring topology operation.
All Catalyst switches connected to an XPS on SP ports are part of the same power stack, and all power from the XPS and the
switches is shared across all switches in the stack. Power sharing is the default mode, but the XPS supports the same stack
power modes that are supported in a ring topology (strict and nonstrict power-sharing or redundant modes).
The XPS uses neighbor discovery to create the power stack. When it discovers a Catalyst switch on an unconfigured port, it
marks the port as an SP port, and the switch joins the power stack. The XPS notifies the switch, begins the power-budgeting
process, and assigns budgets to each switch in the power stack based on their requirements, priorities, current power allocations,
and the stack aggregate power capability.
The XPS sends the power budget to each switch. If not enough input power is available to provide every switch with its maximum
requested power, power is distributed based on priority. Switches with the highest priority receive required power first,
followed by any powered devices that have already been allocated power, in order of their priority. Any remaining power is
distributed equally through the stack.
The RPS port priority (1 through 9) does not affect stack power priority. Each switch participating in stack power has its
own system priority and a high and low priority for devices connected to its ports. These priorities are used for stack power,
as is the case in a ring topology. You configure stack power priority for the system and for high and low-priority ports by
using the power-priority switch , power-priority high , and power-priority low commands in switch stack power configuration mode. If a system or set of powered devices are using the default priority,
the XPS automatically assigns a priority (1 through 27), with lower MAC addresses receiving higher priorities.
There are four power stack modes: power sharing, strict power sharing, redundant, or strict redundant. You configure the power
stack mode by using the mode {power-sharing | redundant} [strict] command in power-stack configuration mode. The power-sharing or redundant configurations affect the power budgeting aspect of the stack; strict or non-strict affects the actions of the PoE application when a budget reduction does not result in load shedding.
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In power sharing modes (strict or nonstrict), the stack power budget is the cumulative capacity of all the power supplies
in the stack (minus 30 W reserved power). This is the default.
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In redundant modes (strict or nonstrict), the stack power budget is the total available power (minus 30 W) after the capacity
of the largest power supply in the power stack is subtracted. Redundant mode guarantees that no switch or powered device loses
power or experiences load sheds if a single power supply fails, but load sheds can occur if more than one power supply fails.
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In strict modes, if a loss of input power results in reduced power budgets but does not result in any hardware load shedding,
the XPS automatically begins denying power to low-priority powered devices and then the high-priority powered devices until
the amount of allocated power is less than or equal to the amount of available PoE power.
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In nonstrict modes, in the event of a power reduction, the amount of allocated power is allowed to fall under budget.
For example, a system with a total PoE budget (available power) of 400 W can allocate 390 W of the budget (allocated power)
to powered devices. The allocated power of a device is the maximum amount of power that the device needs. The actual power
consumption (consumed power) for a set of powered devices is usually not equal to the allocated power. In this example, the
actual power might be approximately 200 W. If a power loss in the stack reduces the available power to 210 W, this amount
is enough to sustain the power being consumed by the powered devices, but less than the worst-case allocated power, which
would put the system under budget. In strict mode, the stack would immediately deny power to powered devices until the allocated power was 210 W or less. In
nonstrict mode, no action is taken, and the state is allowed to persist. In nonstrict mode if the actual power consumption
becomes more than 210 W, this triggers a load shed and can result in the loss of power to all powered devices or switches
with the lowest priority level.