CCNA Voice Study – PoE

Cisco pre-standard PoE verses 802.3af PoE

Cisco pre-standard PoE Process
1. Device connected to switch.

2. Switch sends a Fast Link Pulse (FLP) tone to the device. Only unpowered Cisco pre-standard PoE device will loop the FLP back to the switch.

3. Switch receives pulse back and applies a minimal amount of power (6.3W) to the line.

4. Unpowered device boots and communicates it’s actual power requirements via CDP.

802.3af PoE Process
1. Device connected to switch.

2. Constant small DC current is applied to line.

3. 802.3af device is equipped with a resistor and will return a specific level of resistance on the line.

4. Depending on level of resistance switch knows how much power to send to the device.

802.3af Power Class Power Allocated Actual Power Used
Class 0 15.4W 0.44W to 12.95W
Class 1 4.0W 0.44W to 3.84W
Class 2 7.0W 3.84W to 6.49W
Class 3 15.4W 6.49W to 12.95W

Class 0 devices are designed to be cheap to manufacture basically just requesting that power be sent to them. Too many Class 0 devices can exhaust your switch power supply as all Class 0 devices will be allocated the full 15.4W.

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