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Course 5: LAN SwitchingLesson 2.2 (4 of 6 in this course)19 of 91 in the CCNA series

Power over Ethernet (PoE, PoE+, UPOE, UPOE+)

Powering phones, access points and cameras over the data cable.

Intermediate · 7 min read

PoE (Power over Ethernet) is a family of IEEE 802.3 standards that let a switch or a midspan injector (the power sourcing equipment, PSE) deliver DC power to a device (the powered device, PD) over the same twisted-pair cable that carries its data. The PSE first detects a compliant device and then supplies power within the limits of its class.

In simple terms: PoE lets one network cable carry both data and electricity, so devices like IP phones, access points and cameras don’t need a separate power socket.

Power and data on one cable

IP phones, Wi-Fi access points and security cameras are often mounted where there's no power socket. Power over Ethernet sends DC power down the same twisted-pair cable as the data. The switch (or a midspan injector) is the PSE (power sourcing equipment); the device is the PD (powered device).

PoEPoE+UPOE / 802.3btPoE switch (PSE)budget 740 WIP phone~6 WWi-Fi AP~20 WPTZ camera~50 W
  1. 1. Detection: the switch first checks for a PD's signature resistance (about 25 kΩ), so it never sends power to an ordinary PC.
  2. 2. Classification: the PD tells the switch which power class it needs (LLDP or CDP can then fine-tune it).
  3. 3. Power: the switch supplies power from its overall budget. Each device's draw is subtracted from what's left.

The standards

NameStandardAt the switch portAt the devicePairs
PoEIEEE 802.3af (Type 1)15.4 W12.95 W2
PoE+IEEE 802.3at (Type 2)30 W25.5 W2
UPOE (Cisco)Cisco; matches 802.3bt Type 360 W51 W4
UPOE+ (Cisco)Cisco; matches 802.3bt Type 490 W71.3 W4

The device gets less than the port sends because some power is lost as heat in the cable over 100 m. Higher levels use all four pairs.

The power budget

A switch's power supply limits the total PoE it can deliver. A 48-port switch with a 740 W budget can't give all 48 ports 30 W (1440 W). When the budget runs out, new devices aren't powered (or lower-priority ports are shut off). Plan budgets, and use port priority for critical devices.

Configure and verify

interface GigabitEthernet1/0/10 power inline auto

The default: detect, classify and power a PD. 'power inline never' disables PoE on a port; 'power inline static' reserves power in advance.

Example output · based on Cisco documentation; exact format varies by platform and software version
SW1#show power inline
Module   Available     Used     Remaining
          (Watts)     (Watts)    (Watts)
------   ---------   --------   ---------
1           740.0       73.4       666.6
Interface Admin  Oper       Power   Device              Class Max
                            (Watts)
--------- ------ ---------- ------- ------------------- ----- ----
Gi1/0/1   auto   on         6.3     IP Phone            2     30.0
Gi1/0/2   auto   on         17.1    Access Point        4     30.0
Gi1/0/3   auto   on         50.0    Camera              6     60.0
Gi1/0/4   auto   off        0.0     n/a                 n/a   30.0
The top shows the budget: available, used and remaining. Each port shows whether it's powering something (Oper on), how much it draws and its class. off on Gi1/0/4 means no powered device is connected.

Check yourself

Predict · scenario 1

An AP needs about 25 W at the device. Which is the minimum standard that supports it?

Predict · scenario 2

A new camera won't power up, but its port shows no errors and other ports work. What should you check first?