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CCNA

Understanding Power over Ethernet (PoE)

Power over Ethernet (PoE) allows network switches to deliver both data and electrical power over a single Ethernet cable, eliminating the need for separate power supplies. This post covers the major IEEE PoE standards (802.3af, 802.3at, 802.3bt), how the detection and classification process works,

Jay Whale

• 3 min read
Understanding Power over Ethernet (PoE)

If you've ever wondered how an IP camera or wireless access point gets its power without a separate power cable, the answer is almost certainly Power over Ethernet. PoE is one of those technologies that seems like a small detail until you realize how much it simplifies real-world network deployments.

What Is Power over Ethernet?

Power over Ethernet (PoE) is a technology that allows network switches to deliver electrical power to connected devices over standard Ethernet cabling. Instead of running a separate power cable to each device, a single Ethernet cable carries both data and power simultaneously. This is a significant practical advantage in environments where power outlets are scarce or inconveniently located.

The power sourcing equipment (PSE) is the device that provides power, typically a PoE-capable switch. The device receiving power is called the powered device (PD). Common powered devices include:

  • Wireless access points (WAPs)
  • IP phones
  • IP security cameras
  • Door access control systems
  • IoT sensors

PoE Standards You Should Know

The IEEE has defined several PoE standards over the years, each delivering progressively more power. For the CCNA exam, you should be familiar with the most common ones:

  • IEEE 802.3af (PoE): The original standard, delivering up to 15.4 watts per port at the PSE, with the PD receiving up to 12.95 watts after cable loss.
  • IEEE 802.3at (PoE+): An enhanced standard delivering up to 30 watts per port, suitable for more power-hungry devices like PTZ cameras and dual-radio access points.
  • IEEE 802.3bt (PoE++): The latest standard, delivering up to 60 or 90 watts per port depending on the type, enabling devices like laptops and video conferencing systems to be powered over Ethernet.

When selecting a PoE switch, always verify that the port wattage matches what your powered devices require. A mismatch can result in the device failing to power on or operating in a degraded mode.

How PoE Works at a Basic Level

Before a PoE switch delivers power to a device, it runs a detection process to verify the connected device is a legitimate powered device and not something that could be damaged by receiving unexpected voltage. This detection uses a low-voltage signal to measure the resistance of the connected device.

Once a valid PD is detected, the PSE optionally classifies the device to determine how much power it needs, then begins supplying power over the unused wire pairs (in older implementations) or the data wire pairs (in newer implementations).

This detection and classification process happens automatically. From a configuration standpoint, PoE is largely plug-and-play on Cisco switches, though you can inspect and control it from the CLI.

Verifying PoE on a Cisco Switch

You can check the PoE status of all ports on a Cisco switch using the following command:

Switch# show power inline

Available:370.0(w)  Used:15.4(w)  Remaining:354.6(w)

Interface Admin  Oper       Power   Device              Class Max
                            (Watts)
--------- ------ ---------- ------- ------------------- ----- ----
Gi0/1     auto   on         15.4    IP Phone 8841       3     30.0
Gi0/2     auto   off        0.0     n/a                 n/a   30.0

To check a specific interface in more detail, use:

Switch# show power inline GigabitEthernet 0/1

You can also disable PoE on a specific port if needed, for example to prevent unauthorized devices from drawing power:

Switch(config)# interface GigabitEthernet 0/1
Switch(config-if)# power inline never

And to restore default PoE behavior on that port:

Switch(config-if)# power inline auto

Why PoE Matters for the CCNA

Exam objective 1.1.h asks you to explain the role and function of network components, and PoE is a key part of how modern access layer switches function. Beyond the exam, understanding PoE helps you make informed hardware decisions when designing or troubleshooting a network. A switch with insufficient total PoE budget can silently starve powered devices of power, which is a surprisingly common real-world issue.

What's Next

Now that you understand how PoE delivers network power to endpoint devices, the next logical step is to look at how those devices communicate once they are connected. In the next post, we will cover network topology types and how different physical and logical layouts affect how traffic flows across your infrastructure.

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If you want visibility beyond the CLI, PRTG Network Monitor can poll your PoE switches via SNMP to track per-port power consumption, alert you when devices lose power, and give you a dashboard view of your entire PoE infrastructure. PRTG Network Monitor.

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