📘 CCNA 200-301 v1.1

1.1.h PoE

🔹 What is PoE (Power over Ethernet)

PoE stands for Power over Ethernet.
It is a technology that allows electrical power to be transmitted along with data over a single Ethernet cable (such as Cat5e, Cat6, etc.).

In other words, the same Ethernet cable that carries network data to a device can also supply electrical power to that device.

This eliminates the need for a separate power adapter or electrical outlet near the device.

🔹 Why PoE is Used in Networking

In many network environments, some devices are installed in areas where providing both network connectivity and electrical power separately is inconvenient or expensive.

Common network devices that use PoE include:

IP phones
Wireless access points (WAPs)
IP cameras (security cameras)
Thin clients or network sensors

In these cases, PoE simplifies deployment by using one Ethernet cable for both data and power.

🔹 How PoE Works (Concept Overview)

PoE works by sending DC power over the unused pairs or data pairs in an Ethernet cable, depending on the standard.

A PoE-capable switch (or injector) detects whether a connected device supports PoE and then supplies power safely.

There are two key device roles in a PoE setup:

Role	Full Name	Function
PSE	Power Sourcing Equipment	The device that provides (injects) power into the Ethernet cable. Example: PoE switch or midspan injector.
PD	Powered Device	The device that receives power from the Ethernet cable. Example: IP phone, access point, or IP camera.

🔹 Types of PoE Devices

1. Endspan (PoE Switch)

A switch with built-in PoE capability.
It acts as the PSE and sends both data and power directly over the cable.
Example: A Cisco Catalyst switch with PoE ports.

2. Midspan (PoE Injector)

A device placed between a non-PoE switch and the PoE device.
It adds power to the Ethernet cable, while the switch continues to handle data only.
Used when your switch does not support PoE.

🔹 IEEE PoE Standards (Important for CCNA)

The IEEE defines three main PoE standards that determine how much power can be delivered:

Standard	Name	Max Power (at PSE)	Max Power (at PD)	Typical Use
802.3af	PoE	15.4 W	12.95 W	IP phones, wireless APs
802.3at	PoE+	30 W	25.5 W	More powerful APs, PTZ cameras
802.3bt	PoE++ (Type 3 & 4)	60 W (Type 3) / 100 W (Type 4)	51–71 W	High-end devices, thin clients, LED lighting

💡 Note: The power received (PD) is always less than power sent (PSE) because of cable loss.

🔹 Power Negotiation Process

Before power is sent, a negotiation happens to ensure safety and compatibility:

Detection:
- The PSE (e.g., switch) checks if the connected device is PoE-compatible by sending a small voltage pulse.
Classification:
- The PD reports its power class (how much power it needs).
Power Delivery:
- The PSE supplies the required voltage (usually 48V DC).
Operation:
- The PD uses the power and data simultaneously.
Disconnection:
- If the device is unplugged or stops drawing current, power is automatically turned off to protect the port.

🔹 Power Classes (IEEE Defined)

Each PoE device declares a power class during negotiation.

Class	Power Level	Standard	Typical Power Delivered (PSE)
0	Default	802.3af	15.4 W
1	Low power	802.3af	4 W
2	Medium	802.3af	7 W
3	High	802.3af	15.4 W
4	PoE+	802.3at	30 W
5	PoE++ Type 3	802.3bt	45 W
6	PoE++ Type 3	802.3bt	60 W
7	PoE++ Type 4	802.3bt	75 W
8	PoE++ Type 4	802.3bt	100 W

🔹 PoE in Network Design (CCNA-Relevant)

When designing or configuring networks:

Verify switch capability: Use show power inline on Cisco switches to view PoE usage and available power.
Power budget: Ensure the switch’s total PoE power budget can support all connected devices.
Example: A switch with a 370 W PoE budget can support multiple 15.4 W devices.
Cable quality: Use Cat5e or higher cables to minimize voltage drop and ensure full power delivery.
Backup power: When a switch provides PoE, a UPS (Uninterruptible Power Supply) can keep powered devices (like phones and APs) running during power loss.

🔹 Common Cisco PoE Commands (Know for CCNA Labs)

show power inline
→ Displays PoE status, power used, and available per port.
interface FastEthernet0/1
power inline auto
→ Enables PoE on a specific interface (default on most Cisco PoE switches).
power inline never
→ Disables PoE on a specific port.

🔹 Benefits of PoE

Benefit	Description
Simplified cabling	One cable for both power and data.
Flexible installation	Devices can be installed anywhere Ethernet can reach.
Centralized power management	Power can be controlled from the switch.
Reduced downtime	Switches connected to UPS keep devices powered during outages.

🔹 Limitations of PoE

Limitation	Description
Power distance limit	Max Ethernet cable length: 100 meters (including patch cables).
Total power budget	Each switch has limited total PoE power capacity.
Heat generation	Higher power levels may cause cable heating.

✅ Summary (Exam Essentials)

Key Concept	Description
PoE	Sends data + power through same Ethernet cable.
PSE	Device providing power (PoE switch/injector).
PD	Device receiving power (phone, AP, camera).
Standards	802.3af (PoE), 802.3at (PoE+), 802.3bt (PoE++).
Max Cable Length	100 meters.
Power Classes	Define how much power is needed by the PD.
Commands to remember	`show power inline`, `power inline auto`, `power inline never`.