Information about PoE
Power over Ethernet Ports
A PoE-capable switch port automatically supplies power to one of these connected devices if the switch senses that there is no power on the circuit:
-
a Cisco pre-standard powered device (such as a Cisco IP Phone or a Cisco Aironet Access Point)
-
an IEEE 802.3af-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.
PoE and PoE Pass-Through Ports on Catalyst WS-C3560CX-8PT-S
The Catalyst WS-C3560CX-8PT-S is a PD/PSE product, which means that the switch can behave like both a Power Device (PD) and Power Source Equipment (PSE). This switch will be powered on by the PoE voltage derived from its uplink ports (PD1 or PD2) or from the voltage supplied by external auxiliary power supply (AUX). The switch will enable powering over PoE, PoE+ and UPOE, as well as AC and DC input.
The power available from uplinks and one of the power adapter will be added for increased input power, which translates to a higher PoE budget. Some of this power will be used for system power and rest would be provided to downlink POE+ ports as pass-through power that will be available to power other PoE peripheral devices like IP phones, IP Cameras and so on.
-
The Catalyst WS-C3560CX-8PT-S will support powering from 2xUPOE uplinks.
-
It will support a DC power adaptor which will enable the switch to be powered by 24V DC input.
-
AUX contributes 78W to the system.
-
The power sources (AC or DC) and PoE will be additive. The table below lists different power values for PoE budget.
|
PoE Budget(Watts) |
Uplink 1 |
Uplink 2 |
Comment |
|---|---|---|---|
|
0 |
PoE |
PoE |
Normal operation, no PoE budget |
|
0 |
0 |
PoE+ |
Normal operation, no PoE budget |
|
20 |
PoE+ |
PoE+ |
PoE budget available |
|
22 |
0 |
UPoE |
PoE budget available |
|
33 |
UPoE |
PoE |
PoE budget available |
|
44 |
PoE+ |
UPoE |
PoE budget available |
|
68 |
UPoE |
UPoE |
PoE budget available |
-
One of the uplink port is connected to the PSE.
-
No Auxiliary power adapter is connected.
In this case, the switch will power up in low power mode with ASIC powered down and negotiate power using CDP/LLDP. The system will power up and initialize ASIC once power is negotiated and continue to boot without software reload.
Example: Configuring PoE and PoE Pass-Through Ports on WS-C3560CX-8PT-S
The show env power privileged EXEC command provides information about powering options on your switch:
SwitchDevice# show env power
Power Source Type Power(w) Mode
-------------- -------------- --------- ---------
A.C. Input Auxilliary 80(w) Available
Gi0/9 Type2 30(w) Available
Gi0/10 Type2 30(w) Available
Available : The PoE received on this link is used for powering this switch and
providing PoE pass-through if applicable.
 Note |
All these power sources adds up to the POE budget. The system consumption is approximately 24W. |
Supported Protocols and Standards
The switch uses these protocols and standards to support PoE:
-
CDP with power consumption—The powered device notifies the switch of the amount of power it is consuming. The switch does not reply to the power-consumption messages. The switch can only supply power to or remove power from the PoE port.
-
Cisco intelligent power management—The powered device and the switch negotiate through power-negotiation CDP messages for an agreed-upon power-consumption level. The negotiation allows a high-power Cisco powered device, which consumes more than 7 W, to operate at its highest power mode. The powered device first boots up in low-power mode, consumes less than 7 W, and negotiates to obtain enough power to operate in high-power mode. The device changes to high-power mode only when it receives confirmation from the switch.
High-power devices can operate in low-power mode on switches that do not support power-negotiation CDP.
Cisco intelligent power management is backward-compatible with CDP with power consumption; the switch responds according to the CDP message that it receives. CDP is not supported on third-party powered devices; therefore, the switch uses the IEEE classification to determine the power usage of the device.
-
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.
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:
-
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.
-
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. Table 1 lists these levels.
|
Class |
Maximum Power Level Required from the Switch |
|---|---|
|
0 (class status unknown) |
15.4 W |
|
1 |
4 W |
|
2 |
7 W |
|
3 |
15.4 W |
|
4 |
30 W (For IEEE 802.3at Type 2 powered devices) |
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 switch for PoE). Theswitch 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. This 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, generates a syslog message, and updates the LEDs. 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 initial allocation for Class 0, Class 3, and Class 4 powered devices is 15.4 W. When a device starts up and uses CDP or LLDP to send a request for more than 15.4 W, it can be allocated up to the maximum of 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 and LEDs.
Power Management Modes
The switch supports these PoE modes:
-
auto —The switch automatically detects if the connected device requires power. If the switch discovers a powered device connected to the port and if the switch has enough power, it grants power, updates the power budget, turns on power to the port on a first-come, first-served basis, and updates the LEDs. For LED information, see the hardware installation guide.
If the switch has enough power for all the powered devices, they all come up. If enough power is available for all powered devices connected to the switch, 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 switch denies power, ensures that power to the port is turned off, generates a syslog message, and updates the LEDs. After power has been denied, the switch periodically rechecks the power budget and continues to attempt to grant the request for power.
If a device being powered by the switch is then connected to wall power, the switch might continue to power the device. The switch might continue to report that it is still powering the device whether the device is being powered by the switch or receiving power from an AC power source.
If a powered device is removed, the switch 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 switch does not provide power to the port. If the switch powers a powered device, but the powered device later requests through CDP messages more than the configured maximum value, the switch 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 switch delivers the maximum value. Use the auto setting on any PoE port. The auto mode is the default setting.
-
static —The switch pre-allocates power to the port (even when no powered device is connected) and guarantees that power will be available for the port. The switch 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 switch does not supply power to it. If the switch learns through CDP messages that the powered device is consuming more than the maximum wattage, the switch shuts down the powered device.
If you do not specify a wattage, the switch pre-allocates the maximum value. The switch powers the port only if it discovers a powered device. Use the static setting on a high-priority interface.
-
never —The switch 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 switch 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 switch senses the real-time power consumption of the powered device. The switch monitors the real-time power consumption of the connected powered device; this is called power monitoring or power sensing. The switch 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 switch senses the real-time power consumption of the connected device as follows:
-
The switch monitors the real-time power consumption on individual ports.
-
The switch records the power consumption, including peak power usage. The switch reports the information through the CISCO-POWER-ETHERNET-EXT-MIB.
-
If power policing is enabled, the switch 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 switch can either turn off power to the port, or the switch can generate a syslog message and update the LEDs (the port LED is now blinking amber) while still providing power to the device based on the switch configuration. By default, power-usage policing is disabled on all PoE ports.
If error recovery from the PoE error-disabled state is enabled, the switch 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.
-
If policing is disabled, no action occurs when the powered device consumes more than the maximum power allocation on the PoE port, which could adversely affect the switch.
Maximum Power Allocation (Cutoff Power) on a PoE Port
When power policing is enabled, the switch determines one of the these values as the cutoff power on the PoE port in this order:
-
Manually when you set the user-defined power level that the switch budgets for the port by using the power inline consumption default wattage global or interface configuration command
-
Manually when you set the user-defined power level that limits the power allowed on the port by using the power inline auto max max-wattage or the power inline static max max-wattage interface configuration command
-
Automatically when the switch sets the power usage of the device by using CDP power negotiation or by the IEEE classification and LLDP power negotiation.
Use the first or second method in the previous list to manually configure the cutoff-power value by entering the power inline consumption default wattage or the power inline [auto | static max] max-wattage command.
If you do not manually configure the cutoff-power value, the switch automatically determines it by using CDP power negotiation or the device IEEE classification and LLDP power negotiation. If CDP or LLDP are not enabled, the default value of 30 W is applied. However without CDP or LLDP, the switch does not allow devices to consume more than 15.4 W of power because values from 15400 to 30000 mW are only allocated based on CDP or LLDP requests. If a powered device consumes more than 15.4 W without CDP or LLDP negotiation, the device might be in violation of the maximum current (Imax) limitation and might experience an Icut fault for drawing more current than the maximum. The port remains in the fault state for a time before attempting to power on again. If the port continuously draws more than 15.4 W, the cycle repeats.
Note |
When a powered device connected to a PoE+ port restarts and sends a CDP or LLDP packet with a power TLV, the switch locks to the power-negotiation protocol of that first packet and does not respond to power requests from the other protocol. For example, if the switch is locked to CDP, it does not provide power to devices that send LLDP requests. If CDP is disabled after the switch has locked on it, the switch does not respond to LLDP power requests and can no longer power on any accessories. In this case, you should restart the powered device. |
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 switch 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 switch uses for power policing is not equal to the configured power value.
When power policing is enabled, the switch polices the power usage at the switch port, which is greater than the power consumption of the device. When you are 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 switch. For example, 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 switch 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 switch does not provide power to the connected device. After the switch turns on power on the PoE port, the switch 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 switch and the devices connected to the other PoE ports.
Because the switch supports internal power supplies and the Cisco Redundant Power System 2300 (also referred to as the RPS 2300), the total amount of power available for the powered devices varies depending on the power supply configuration.
Feedback