- Power sourcing equipment (PSE)—A device that provides power through a twisted-pair Ethernet connection. The switch, through switching modules equipped with Power over Ethernet (PoE) daughtercards, functions in the PSE role.
- Powered device (PD)—A device powered by a PSE (for example, IP phones, IP cameras, and wireless access points).
Note Not all PoE-capable devices are powered from the switch. There are two sources of local power for PoE-capable devices:
- A power supply connected to the device.
- A power supply through a patch panel over the Ethernet connection to the device.
When a locally powered PoE-capable device is present on a switching module port, the switching module itself cannot detect its presence. If the device supports CDP, the supervisor engine can discover a locally powered PoE-capable device through CDP messaging with the device. If a locally powered PoE-capable device loses local power, the switching module can discover and supply power to the IP phone if the inline power mode is set to auto.
Cisco PoE daughtercards support one or more PoE implementation:
- IEEE 802.3at standard, shown in Cisco Feature Navigator as “PoE Plus (PoE+, PoEP) support”.
– Supported only with the PoE daughtercard on the WS-X6148E-GE-45AT switching module.
– These features are supported for IEEE 802.3at-compliant class 4 PDs:
• Class 4: 30.00 W at the PSE (12.95 W to 25.50 W at the PD).
• Optionally, LLDP Inline Power Negotiation for PoE+.
– With releases earlier than Release 15.1(1)SY, maximum 16.8 W at the PSE (ePoE for 45 ports maximum).
– Supported with the WS-F6K-48-AF PoE daughtercard and the PoE daughtercard on the WS-X6148E-GE-45AT switching module.
– Maximum 16.80 W at the PSE.
– The IEEE 802.3af PoE standard defines a method to sense a PD and to immediately classify the power requirement of the PD into these per port power ranges at the PSE:
• Class 0: Up to 15.4 W (0.44–12.95 W at the PD; default classification)
• Class 1: Up to 4 W (0.44–3.84 W at the PD)
• Class 2: Up to 7 W (3.84–6.49 W at the PD)
• Class 3: Up to 15.4 W (6.49–12.95 W at the PD)
- Cisco prestandard inline power—10 W at the PSE.
With a PoE daughtercard installed, a switching module can automatically detect and provision a PoE-capable device that adheres to a PoE implementation supported by the PoE daughtercard. The switching module can supply power to devices supporting other PoE implementations only through manual configuration.
Only a PD connected directly to the switch port can be powered from the switch. If a second PD is daisy-chained from the PD that is connected to the switch port, the second PD cannot be powered by the switch.
Each PD requires power to be allocated from the chassis power budget. Because each PD can have unique power requirements, more devices can be supported if the system’s power management software can intelligently allocate the necessary power on a per-port basis.
You can configure ports to allocate power at a level based on the following:
- If a PD is detected, with auto mode configured:
– Information sensed from the device
– A default level
– A configured maximum level
- Whether or not a PD is present on the port, with static mode configured:
– A default level
– A configured level
CPD-Based PoE Management
When a switching module port detects an unpowered PD, the default-allocated power is provided to the port. When the correct amount of power is determined through CDP messaging with the PD, the supervisor engine reduces or increases the allocated power, up to the hardware limit of the installed PoE daughtercard.
When a PD cable is plugged into a port and the power is turned on, the supervisor engine has a 4-second timeout waiting for the link to go up on the line. During those 4 seconds, if the IP phone cable is unplugged and a network device is plugged in, the network device could be damaged. We recommend that you wait at least 10 seconds between unplugging a network device and plugging in another network device.
Inline Power IEEE Power Classification Override
The IEEE 802.3af standard contains no provision for adjustment of the power allocation. 802.3af-compliant PDs that support CDP can use CDP to override the IEEE 802.3af power classification.
The WS-F6K-48-AF PoE daughtercard or the PoE daughtercard on the WS-X6148E-GE-45AT switching module support these inline power IEEE 802.3af power classification override features:
- Power use measurement—The ability to accurately measure the power provided by the port to the powered device.
- Power policing—The ability to monitor power usage on a port.
With power measurement and policing, you can safely override the IEEE 802.3af power classification of a device that requires a power level at the lower end of its IEEE power classification range.
PoE monitoring and policing compares the power consumption on ports with the administrative maximum value (either a configured maximum value or the port’s default value). If the power consumption on a monitored port exceeds the administrative maximum value, the following actions occur:
- A syslog message is issued.
- The monitored port is shut down and error-disabled.
- The allocated power is freed.
LLDP Inline Power Negotiation for PoE+ (IEEE 802.3at)
The PoE daughtercard on the WS-X6148E-GE-45AT switching module supports IEEE 802.3at -compliant LLDP PoE power negotiation, which supports additional negotiation that can reduce power usage.
Note ● Enabled by default.
- The LLDP TLV used is DTE Power-via-MDI TLV.
- When a PD that performs power negotiation using multiple protocols (CDP and LLDP 802.3at) is connected to a switch, the switch locks to the first protocol packet (CDP or LLDP) that contains the power negotiation TLV. If you need to use any single protocol for power negotiation each time, you must administratively disable the other power negotiation protocols on the switch interface.
- See this publication for other the Link Layer Discovery Protocol (LLDP) configuration procedures: