In a standalone switch, the total power available for powered devices depends on the power supply configuration and the installed internal power supplies.

In a standalone switch, replacing the power supply can affect the available power for the powered device in the following ways:

• If the replacement power supply supports less power, the switch may not have enough power for all powered devices. In this case, the switch denies power to PoE ports, configured in auto mode, starting with the highest port numbers. If there is still insufficient power, the switch denies power to the PoE ports configured in static mode, starting with the lowest port numbers.

• If the replacement power supply supports more power than the previous one, the switch will have additional power available. In this case, the switch grants power to PoE ports configured in static mode, starting with the lowest port numbers. If there is still sufficient power, the switch grants power to the PoE ports configured in auto mode, starting with the highest port numbers.

PoE operates the same whether or not the switch is configured as a standalone or as a stack. The power budget is per switch and independent of the switch members in the stack. Election of a new active switch does not affect PoE operation. The active switch keeps track of the PoE status for all switch members and ports in the stack and includes the status in output displays.

A stacking-capable switch also supports StackPower, which allows the switch's power supplies to share the load across multiple systems in a stack when you connect the switch with power stack cables. You can manage the power supplies of up to four stack members as a single large power supply.

The stacking-capable switch also supports StackPower. StackPower allows the power supplies to share the load across the stack when you connect up to four switch members with power stack cables.

Power management protocols determine how a PoE-capable switch manages power to a powered device. The supported power management protocols are

• CDP with power consumption: The powered device notifies the PoE-capable switch of its power consumption. The switch does not reply to the power consumption messages. The switch can only supply or remove power from the PoE port.

  CDP with power consumption is supported only on Cisco devices.

  Note

  In this method, high-powered devices operate in low-power mode only.

• Cisco intelligent power management: The powered device and the PoE-capable switch use CDP or LLDP messages to negotiate an agreed-upon power-consumption level. The process allows a high-powered device to operate at its highest power mode.

  The powered device first boots up in low-power mode. It then negotiates for additional power to operate in high-power mode. The device switches to high-power mode only after receiving confirmation from the switch.

  Cisco intelligent power management is supported on the following devices:

  • Cisco devices with CDP or LLDP messages

  • third party devices with LLDP messages except in Cisco UPOE mode.

Power management with CDP messages

When a powered devices connects to a PoE port on a switch,

1. the device sends CDP messages to the switch, specifying its precise power consumption requirements.

2. the switch:

   • Receives the CDP messages and determines the exact power requirements of the powered device.

   • Adjusts the power allocation for the port accordingly, ensuring that the device gets exactly the amount of power it requires—no more, no less.

   • Updates the power budget. The switch subtracts the allocated power from its total available power budget, ensuring it can supply power to additional PoE devices without overloading.

Power management with LLDP messages

When a powered devices connects to a PoE port on a switch,

1. The switch uses LLDP messages to discover the device and its capabilities.

2. The powered device sends power-via-MDI TLVs via LLDP to request a specific amount of power (e.g., 25 W). The request includes a proprietary TLV that specifies the

   • Type: The type of power being requested (e.g., spare-pair power for 4-wire delivery).

   • Length: The length of the TLV message.

   • Value: The details of the power requirement (e.g., 30 W or higher).

3. The switch evaluates the request, adjusts its power allocation, and supplies the requested power.

4. If the PD’s power requirements change, updated TLVs are sent, and the switch dynamically adjusts the power delivery.

Power management with Cisco UPOE

When a powered device connects to a Cisco UPOE-enabled switch,

• the switch detects the device and initiates power negotiation using either CDP or LLDP.

• If the device requires more than the standard PoE+ power limit (30 W), it sends an LLDP or CDP message specifying its power needs.

  • Upon receiving the message, the UPOE switch determines whether it can meet the power request. If sufficient power is available, the switch activates the spare pairs (in addition to the signal pairs) to deliver up to 60 W of power.

  • The switch begins supplying power over all four pairs of wires, enabling the powered device to operate in high-power mode.

• If the device does not support CDP or LLDP messages, but it supports detection and classification on both signal and spare pairs, you can configure a 4-pair forced mode. This mode automatically enables power on both signal and spare pairs from the switch port.