JUNOS Enterprise Switching

Power over Ethernet

We discussed PoE capabilities for different EX Series platforms in Chapter 2. Recall that PoE is a standard defined in 802.3af that delivers power over standard copper Ethernet cable that is also delivering data. This technology allows various devices such as Wireless Access Points (WAPs), video cameras, and point-of-sale devices to be powered. Most importantly for this chapter, PoE supplies power for many IP phones.

PoE defines two type of devices: power sourcing equipment (PSE) and powered devices (PDs). For example, a PSE is a switch that provides power to a PD such as an IP phone.

The PoE standard defines different classes of attached devices that can require different levels of power. This allows for better utilization and distribution of power capacity. The classes are:

Class 0

15.4 watts reserved

Class 1

4 watts reserved

Class 2

7 watts reserved

Class 3

15.4 watts reserved

Class 4

Reserved for future expansion

Note

Line loss can reduce the total amount of received power by up to 15% or 16%.

The default class is 0, and is used for any device that does not have a class defined. Also of interest is the fact that Class 0 and Class 3 both have 15.4 watts reserved. The primary difference is that only a device that supports 802.3af can be a Class 3 PD, whereas any device can be a Class 0 PD.

When configuring a PoE device, you may have a limited amount of total system power and want to distribute that power accordingly. This is often referred to as power management or a power ...