How to check the realtime data of the power inline consumption of a Power over Ethernet (PoE) device connected to Catalyst 3750 Series Switches

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Fri, 02/25/2011 - 01:34
Jun 22nd, 2009
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Core issue

This can be required in a scenario where you use third party IP phones that draw too much PoE. Hence, if there is a show or debug command that can be done to see real-time data or data averaged over a certain period of time on the power consumption of a POE device that is connected to a Cisco switch.

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). 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 Cisco Discovery Protocol (CDP) to determine the actual power consumption requirement of the connected Cisco powered devices, and 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.

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.

The syslog message looks similar to this output:

01:13:11: %ILPOWER-7-DETECT: Interface Fa0/10: Power Device detected: IEEE PD
01:13:11: Ilpower PD device 3 class 3 from interface (Fa0/10)
01:13:11: Ilpower deny interface(Fa0/10) because request power 15400 is greater than user config max 7500.

The 15400 is the amount of power requested by the device and the 7500 is the configured amount of power the switch can provide to the device connected to the port.

In this case, if the connected devices are non-Cisco then you are not able to use CDP. Therefore, if the device requests more power, then it is denied.


Resolution

The hardware cannot measure real time power consumption. It is whatever power the device asked for initially (for which the switch is budgeted).

  • IEEE Device - based on class
  • Cisco Device - based on CDP message
       

The device might actually be drawing more or less than the power allotted. The switch cannot monitor this. It only knows about the configured power.

The only option is to display the PoE status for the specified PoE port or for all PoE ports with the show power inline user EXEC command.

For more information on how to configure the Power Inline Consumption feature in Cisco Catalyst 3560 and 3750 Series Switches, refer to the article How to configure the Power Inline Consumption feature in Cisco Catalyst 3560 and 3750 Series switches.

For more information, refer to the Configuring a Power Management Mode on a PoE Port section of Configuring Interface Characteristics.

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