Information About PoE
The following sections provide information about Power over Ethernet (PoE), the supported protocols and standards, and power management.
PoE and PoE+ Ports
A PoE-capable switch port automatically supplies power to one of these connected devices if the device senses that there is no power on the circuit:
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A Cisco prestandard powered device (such as a Cisco IP Phone)
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An IEEE 802.3af-compliant powered device
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An IEEE 802.3at-compliant powered device
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An IEEE 802.3bt-compliant powered device
A powered device can receive redundant power when it is connected to a PoE switch port and to an AC power source. The device does not receive redundant power when it is only connected to the PoE port.
Supported Protocols and Standards
The device uses the following protocols and standards to support PoE:
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CDP with power consumption: The powered device notifies the device of the amount of power it is consuming. The device does not reply to the power-consumption messages. The device can only supply power to or remove power from the PoE port.
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Cisco intelligent power management: The powered device and the device negotiate through power-negotiation CDP messages for an agreed-upon power-consumption level. The negotiation allows a Cisco powered device, that requires different power levels than its current allocation, to operate. The powered device first boots with its IEEE class power or 15.4 W (pre-standard Cisco PD), then negotiates power to operate at appropriate power level. The device consumption changes to requested power mode only when it receives confirmation from the device.
High-power devices can operate in low-power mode on device that do not support power-negotiation CDP.
Cisco intelligent power management is backward-compatible with CDP with power consumption; the device responds according to the CDP message that it receives. CDP is not supported on third-party powered devices; therefore, the device uses the IEEE classification to determine the power usage of the device.
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IEEE 802.3af: The major features of this standard are powered-device discovery, power administration, disconnect detection, and optional powered-device power classification. For more information, see the standard.
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IEEE 802.3at: The PoE+ standard increases the maximum power that can be drawn by a powered device from 15.4 W per port to 30 W per port.
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The Cisco UPOE feature provides the capability to source up to 60 W of power (2 x 30 W) over both signal and spare pairs of the RJ-45 Ethernet cable by using the Layer-2 power negotiation protocols such as CDP or LLDP. An LLDP and CDP request of 30 W and higher in presence of the 4-wire Cisco Proprietary spare-pair power TLV can provide power on the spare pair.
When enabled in IEEE 802.3bt mode, Cisco UPOE devices function as 802.3bt Type 3 devices, supporting upto Class 6 (refer the IEEE Power Classification table in the document) on every port.
Note
Only the following UPOE linecards are IEEE 802.3bt-complaint Type 3 devices:
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C9400-LC-48U
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C9400-LC-48UX
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IEEE 802.3bt: IEEE 802.3bt standard enables delivery of up to 90 W to a powered device, over four pairs of Category 5e and above cables. This standard also introduces additional classes of Power Sourcing Equipments (PSEs) and powered devices, class 5 to class 8, with PSE output power ranging between 45 W to 90 W and the powered device input power ranging from 40 W to 71.3 W. It introduces new types of PSEs or powered devices: Type 3(60 W) and Type 4 (90 W). IEEE 802.3bt standard enables support for Dual Signature Powered Devices, Single Signature Powered Devices, and Single Pair Powered Devices. It also supports power demotion to handle scenarios where Type 4 Powered Device is connected to a Type 3 PSE.
For more information on IEEE 802.3bt, see the standard.
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Cisco UPOE+: Cisco UPOE+ feature provides the capability to source up to 90 W on the IEEE 802.3bt-compliant Type 4 devices.
A Type 3 PSE can power up a Type 4 powered device through a power demotion to 60 W.
Cisco IOS XE Release 16.12.1 introduces C9400-LC-48H, an 802.3bt-compliant Type 4 device.
Some legacy Cisco powered devices (like 7910, 7940, 7960 IP phones and AP350 wireless access points) are incompatible with Type 4 Power Supply Equipments (PSEs), as defined in the IEEE 802.3bt standard. If connected, the PSE will report a ‘Tstart’ or ‘Imax’ fault with each periodic attempt at providing power to the powered device. For continued use of these legacy Cisco powered devices, connect them to Cisco PoE+/UPOE PSEs.
Powered devices which do not meet the standard detection signature capacitance (such as CIVS-IPC-6000P) may be detected properly with POE+ or Cisco UPOE devices running in UPOE mode, but may not be detected properly when running in 802.3bt mode.
Powered-Device Detection and Initial Power Allocation
The switch detects a Cisco pre-standard or an IEEE-compliant powered device when the PoE-capable port is in the no-shutdown state, PoE is enabled (the default), and the connected device is not being powered by an AC adaptor.
After device detection, the switch determines the device power requirements based on its type:
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The initial power allocation is the maximum amount of power that a powered device requires. The switch initially allocates this amount of power when it detects and powers the powered device. As the switch receives CDP messages from the powered device and as the powered device negotiates power levels with the switch through CDP power-negotiation messages, the initial power allocation might be adjusted.
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The switch classifies the detected IEEE device within a power consumption class. Based on the available power in the power budget, the switch determines if a port can be powered. The following IEEE Power Classifications table lists these levels.
Class |
Maximum Power Level Required from the Device |
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0 (class status unknown) |
15.4 W |
1 |
4 W |
2 |
7 W |
3 |
15.4 W |
4 |
30 W |
5 |
45 W |
6 |
60 W |
7 |
75 W |
8 |
90 W |
The switch monitors and tracks requests for power and grants power only when it is available. The switch tracks its power budget (the amount of power available on the device for PoE). The switch performs power-accounting calculations when a port is granted or denied power to keep the power budget up to date.
After power is applied to the port, the switch uses CDP to determine the CDP-specific power consumption requirement of the connected Cisco powered devices, which is the amount of power to allocate based on the CDP messages. The switch adjusts the power budget accordingly, through CDP or LLDP. Note that CDP does not apply to third-party PoE devices. The switch processes a request and either grants or denies power. If the request is granted, the switch updates the power budget. If the request is denied, the switch ensures that power to the port is turned off, and generates a syslog message. Powered devices can also negotiate with the switch for more power.
With PoE+, powered devices use IEEE 802.3at and LLDP power with media dependent interface (MDI) type, length, and value descriptions (TLVs), Power-via-MDI TLVs, for negotiating power up to 30 W. Cisco pre-standard devices and Cisco IEEE powered devices can use CDP or the IEEE 802.3at power-via-MDI power negotiation mechanism to request power levels up to 30 W.
Note |
The CDP-specific power consumption requirement is referred to as the actual power consumption requirement in the software configuration guides and command references. |
If the switch detects a fault caused by an undervoltage, overvoltage, overtemperature, oscillator-fault, or short-circuit condition, it turns off power to the port, generates a syslog message, and updates the power budget.
Power Management Modes
The device supports these PoE modes:
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auto : The device automatically detects if the connected device requires power. If the device discovers a powered device connected to the port and if the device has enough power, it grants power, updates the power budget, turns on power to the port on a first-come, first-served basis.
If the device has enough power for all the powered devices, they all come up. If enough power is available for all powered devices connected to the device, power is turned on to all devices. If there is not enough available PoE, or if a device is disconnected and reconnected while other devices are waiting for power, it cannot be determined which devices are granted or are denied power.
If granting power would exceed the system power budget, the device denies power, ensures that power to the port is turned off, and generates a syslog message. After power has been denied, the device periodically rechecks the power budget and continues to attempt to grant the request for power.
If a device being powered by the device is then connected to wall power, the device might continue to power the device. The device might continue to report that it is still powering the device whether the device is being powered by the device or receiving power from an AC power source.
If a powered device is removed, the device automatically detects the disconnect and removes power from the port. You can connect a nonpowered device without damaging it.
You can specify the maximum wattage that is allowed on the port. If the IEEE class maximum wattage of the powered device is greater than the configured maximum value, the device does not provide power to the port. If the device powers a powered device, but the powered device later requests through CDP messages more than the configured maximum value, the device removes power to the port. The power that was allocated to the powered device is reclaimed into the global power budget. If you do not specify a wattage, the device delivers the maximum value. Use the auto setting on any PoE port. The auto mode is the default setting.
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static : The device pre-allocates power to the port (even when no powered device is connected) and guarantees that power will be available for the port. The device allocates the port configured maximum wattage, and the amount is never adjusted through the IEEE class or by CDP messages from the powered device. Because power is pre-allocated, any powered device that uses less than or equal to the maximum wattage is guaranteed to be powered when it is connected to the static port. The port no longer participates in the first-come, first-served model.
However, if the powered-device IEEE class is greater than the maximum wattage, the device does not supply power to it. If the device learns through CDP messages that the powered device is consuming more than the maximum wattage, the device shuts down the powered device.
If you do not specify a wattage, the device pre-allocates the maximum value. The device powers the port only if it discovers a powered device. Use the static setting on a high-priority interface.
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never : The device disables powered-device detection and never powers the PoE port even if an unpowered device is connected. Use this mode only when you want to make sure that power is never applied to a PoE-capable port, making the port a data-only port.
For most situations, the default configuration (auto mode) works well, providing plug-and-play operation. No further configuration is required. However, perform this task to configure a PoE port for a higher priority, to make it data only, or to specify a maximum wattage to disallow high-power powered devices on a port.
Power Monitoring and Power Policing
When policing of the real-time power consumption is enabled, the device takes action when a powered device consumes more power than the maximum amount allocated, also referred to as the cutoff-power value.
When PoE is enabled, the device senses the real-time power consumption of the powered device. The device monitors the real-time power consumption of the connected powered device; this is called power monitoring or power sensing. The device also polices the power usage with the power policing feature.
Power monitoring is backward-compatible with Cisco intelligent power management and CDP-based power consumption. It works with these features to ensure that the PoE port can supply power to the powered device.
The device senses the real-time power consumption of the connected device as follows:
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The device monitors the real-time power consumption on individual ports.
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The device records the power consumption, including peak power usage. The device reports the information through the CISCO-POWER-ETHERNET-EXT-MIB.
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If power policing is enabled, the device polices power usage by comparing the real-time power consumption to the maximum power allocated to the device. The maximum power consumption is also referred to as the cutoff power on a PoE port.
If the device uses more than the maximum power allocation on the port, the device can either turn off power to the port, or the device can generate a syslog message while still providing power to the device based on the device configuration. By default, power-usage policing is disabled on all PoE ports.
If error recovery from the PoE error-disabled state is enabled, the device automatically takes the PoE port out of the error-disabled state after the specified amount of time.
If error recovery is disabled, you can manually re-enable the PoE port by using the shutdown and no shutdown interface configuration commands.
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If policing is disabled, the powered device can draw a maximum power based on what is allocated by the PSE. If the powered device consumes more than what is allocated, the port hits an Imax error and enters a fault condition.
Power Consumption Values
You can configure the initial power allocation and the maximum power allocation on a port. However, these values are only the configured values that determine when the device should turn on or turn off power on the PoE port. The maximum power allocation is not the same as the actual power consumption of the powered device. The actual cutoff power value that the device uses for power policing is not equal to the configured power value.
When power policing is enabled, the device polices the power usage at the switch port, which is greater than the power consumption of the device. When you manually set the maximum power allocation, you must consider the power loss over the cable from the switch port to the powered device. The cutoff power is the sum of the rated power consumption of the powered device and the worst-case power loss over the cable.
We recommend that you enable power policing when PoE is enabled on your device. For example, for a Class 1 device, if policing is disabled and you set the cutoff-power value by using the power inline auto max 6300 interface configuration command, the configured maximum power allocation on the PoE port is 6.3 W (6300 mW). The device provides power to the connected devices on the port if the device needs up to 6.3 W. If the CDP-power negotiated value or the IEEE classification value exceeds the configured cutoff value, the device does not provide power to the connected device. After the device turns on power on the PoE port, the device does not police the real-time power consumption of the device, and the device can consume more power than the maximum allocated amount, which could adversely affect the device and the devices connected to the other PoE ports.
Cisco Universal Power Over Ethernet
Cisco Universal Power Over Ethernet (Cisco UPOE) is a Cisco proprietary technology that extends the IEEE 802.3 at PoE standard to provide the capability to source up to 60 W of power over standard Ethernet cabling infrastructure (Class D or better) by using the spare pair of an RJ-45 cable (wires 4,5,7,8) with the signal pair (wires 1,2,3,6). Power on the spare pair is enabled when the switch port and end device mutually identify themselves as Cisco UPOE-capable using CDP or LLDP and the end device requests for power to be enabled on the spare pair. When the spare pair is powered, the end device can negotiate up to 60 W of power from the switch using CDP or LLDP.
If the end device supports detection and classification on both signal and spare pairs, but does not support the CDP or LLDP extensions required for Cisco UPOE, a 4-pair forced mode configuration automatically enables power on both signal and spare pairs from the switch port.
You can upgrade the Cisco UPOE devices (C9400-LC-48U and C9400-LC-48UX) to support 802.3bt standard as a Type 3 power sourcing device. A device can support Cisco UPOE and 802.3bt Type 3 on the same port. Note that, while an 802.3bt-compliant Type 3 device and a Cisco UPOE device, both offer 60 W, they operate differently. 802.3bt-compliant devices mutually identify the maximum power requirements during physical classification (refer IEEE Power Classification table). An 802.3bt-compliant Type 3 powered device cannot ask for more power over LLDP than what is requested over the physical layer; which means, an 802.3bt-compliant Class 4 powered device cannot ask for more than 30 W using CDP or LLDP. Whereas, an 802.3bt-compliant Class 6 powered device requests 60 W from the physical layer immediately, before the data link layer is established.
In essence, 802.3at devices support ALT-A (signal pair) 30 W. Cisco UPOE devices support up to 60 W through CDP or LLDP negotiation. 802.3bt-compliant Type 3 4-pair devices can support up to 60 W across Alt-A and Alt-B (both pairs of wires) directly from the physical classification. 802.3bt-compliant Cisco UPOE PSEs support a powered device as physically requested. In addition, Cisco UPOE PSEs continue to support UPOE powered devices. When you upgrade a Cisco UPOE device to 802.3bt mode, there is no change in behaviour of the UPOE powered devices connected to the upgraded PSE.