Power over Ethernet (PoE) is the ability for the LAN switching
infrastructure to provide power over a copper Ethernet cable to an endpoint or
This capability was developed and first delivered by Cisco in 2000 in
order to support the emerging IP Telephony deployments. IP telephones, such as
desktop PBX phones, need power for their operation, and PoE enables scalable
and manageable power delivery and simplifies deployments of IP Telephony.
While IP telephones and wireless access points (APs) are the most
intuitive uses for PoE, the advent of the 802.3af standardization of PoE opens
the door to a new generation of network-attached devices, such as video
cameras, point-of-sale devices, security access control devices (card
scanners), building automation and industrial automation.
PoE promises to create a new world of networked appliances as it
provides power and data connectivity over existing Ethernet cables.
This document answers some of the most frequently asked questions about
Cisco IP phone power requirements.
What is Power over Ethernet?
A. Power over Ethernet (PoE) is the ability to deliver 48 VDC of power
over the same copper cable as Ethernet. Two primary elements are required in
order to implement PoE. They are:
What is the difference between inline power and PoE?
A. They are the same. When Cisco Systems® first introduced powered
Ethernet ports, the technology was called inline power. In order to allow for
universal terminology, Cisco now uses "Power over Ethernet" or "PoE" for all
deployments, standard or pre-standard.
What is the difference between the Cisco original PoE products and the
IEEE 802.3af standard?
A. The differences include:
the amount of power that is available to the connected
the method used for device discovery
the way that power is removed from the wire when a powered device is
What IP phone models support the Cisco pre-standard method of powered
A. These Cisco IP phones can accept Cisco pre-standard PoE from a card
integrated with a Cisco Catalyst switch or a Catalyst in-line power patch
7910G + SW
These phones can draw local power from a power cube (CP-PWR-CUBE-2=) in
addition to a country or regionally specific power cord (CP-PWR-CORD-xx=).
What IP phone models support the IEEE standard method of powered device
A. These Cisco IP phones support IEEE 802.3af PoE:
Note: These Cisco IP phones support both the Cisco pre-standard PoE and
IEEE 802.3af PoE:
Can a Cisco switch be forced to provide pre-standard PoE to an
802.3af-compliant IP phone?
A. There is no way to force the switch to provide pre-standard PoE,
because the power allocation is done automatically through negotiation.
Cisco switches with PoE capability automatically supply power to
connected pre-standard powered devices, such as Cisco IP phones and Cisco
Aironet access points, and to IEEE 802.3af-compliant powered devices if the
switch senses that there is no power on the circuit. This means the switch
supplies power to any non-Cisco device that does not have Cisco Discovery
Protocol (CDP), as long as it is an IEEE 802.3af-compliant powered
In conclusion, Cisco pre-standard PoE devices and 802.3af-compliant
devices work accordingly, and the switch cannot provide either pre-standard PoE
to an 802.3af device or 802.3af power to a Cisco pre-standard device.
Does the Catalyst 3750 switch support Cisco pre-standard compliant IP
A. The Catalyst 3750 switch supports both the Cisco pre-standard PoE
method and the IEEE 802.3af PoE standard. The switches automatically supply
power to connected pre-standard powered devices, such as Cisco IP phones and
Cisco Aironet access points, and to IEEE 802.3af-compliant powered devices if
the switch senses that there is no power on the circuit.
However, there can be issues when you connect some third party
pre-standard compliant devices to the Catalyst 3750, because the third party
devices can potentially use different pins in order to detect power. When you
deal with third party pre-standard compliant devices, check with the
manufacturer about the alignment of the pins for power detection.
How do I determine if the IP phones that receive inline power use the
Cisco pre-standard PoE version or the IEEE 802.3af
A. Cisco Standards based Power over Ethernet supplies power to Cisco IP
phones, Cisco Wireless access points and any third party IEEE 802.3af standard
compliance powered devices (PDs) with the same Ethernet cable that carries the
data. Cisco Catalyst 3750 and 3560 Power over Ethernet supports both the Cisco
pre-standard Power over Ethernet implementation as well as the IEEE 802.3af
Power over Ethernet implementation. This ensures backward and forward
compatibility and investment protection.
The Catalyst 3750/3560 PoE switches support both the Cisco pre-standard
Power over Ethernet, and the Standards based Power over Ethernet methods of PD
detection. Both detection methods are active at the same time, and either one
can be used in order to detect a valid PD. The Catalyst 3750/3560 PoE switches
periodically check all ports, powered and non-powered, in order to check their
status and the power status of connected devices.
Cisco Catalyst 3750/3560 PoE switches support Cisco pre-standard PD
detection mechanisms, and any Standards based compliant PDs. Most Cisco made
PDs, pre-standard or standard, support Cisco Discovery Protocol (CDP). Once
power is applied to a port that contains a pre-standard or standard Cisco PD,
CDP is used in order to determine the actual power requirement, and the system
power budget is adjusted accordingly.
For Cisco pre-standard PDs, if CDP is enabled on the switch, 15.4W is
initially allocated, and then further refined when the CDP message is received
from the PD. If CDP is disabled on the switch, or if the PD does not support
the Power requirements field of the CDP message, the initial allocation value
of 15.4W is used throughout the duration of the connection.
For Standards based compliant PDs, the Catalyst 3750/3560 Power over
Ethernet controller classifies the PD at the detection stage and allocates a
required power budget based on the IEEE class. If a PD supports both IEEE
802.3af and Cisco pre-standard, the PD is detected as an IEEE device. The
Catalyst 3750/3560 PoE switches classify the PD at the detection stage and
allocate a required power budget based on the IEEE class. Then, a CDP message
determines the actual power usage for the PD, provided that CDP is enabled on
the switch. If the requested power through CDP is higher than the PoE
controller classified power, the requested power is adjusted to the PoE
controller IEEE class.
Since all of this happens automatically, it is not possible to
determine whether the IEEE standard or the pre-standard runs.
A Catalyst 3560 switch with 48 ports supports 370W. Because C7941G-GE is
a Class 3 device, it requires up to 15.4W. Can this be reduced to 7W so that
the switch can power all 48 phones?
A. If Cisco Discovery Protocol (CDP) is enabled, there is no need to
reduce the power requirement to 7W. The phone is classified as a Class 3 device
when it first powers up, but after it powers up, CDP sets the desired power
level on the 3560 to 7W. This allows the switch to support 48 ports of
Note: If you use C7941G-GE, it is not possible to power all 48 phones.
C7941G-GE usually draws 12.9W. The total power available is 370W, and for 48
ports, this evenly divides up to ~7.71W per port. In this case, the 3560 switch
can only support 28 phones that draw 12.9W each.
When the pre-standard PoE is used, why does the 7970G IP phone negotiate
15.4W with the 802.3af protocol?
A. A Cisco IEEE+CDP powered device, such as a Cisco IP phone 7970G, comes
up in low power mode (6.3W) and transmits a Cisco Discovery Protocol (CDP)
message with an inline power (ILP) type length value (TLV) that informs the
Power Source Equipment (PSE) of the actual power required by the device. If the
power is less than the default 15.4W, the PSE acknowledges the request with its
available power and modifies the PSEs power budget. If the requesting powered
device exceeds the power budget for the line card or switch, the port is either
powered down, or the port remains in low power mode (7W).
This management scheme is implemented in order to provide backward
compatibility and investment protection to the installed base of Cisco Catalyst
pre-standard Power over Ethernet capable line cards and switches. Cisco IP
phones are power efficient and require 6.3W maximum power as reflected within
the pre-standard Power over Ethernet implementation. However, the development
of new high-power powered devices, such as wireless access points and IP phones
with color LCD screens, requires additional power that cannot be delivered with
the pre-standard implementations. Because Cisco powered devices are brought up
in low power mode, Cisco high-power powered devices can operate, albeit with
reduced functionality, on two pre-standard line cards. Additionally, as Cisco
powered devices explicitly signal their exact power requirements to the PSE,
the PSE can accurately budget power consumption because only the power actually
required by the powered device is allocated.
This management intelligence allows better power resource allocation,
because powered devices can return unused power to the PSE power budget. For
example, if an IEEE 802.3af Class 3 powered device requires 9W, the PSE must
budget for the full 15.4W even though the device only ever draws 9W. This
wastes 6.4W on the powered device. If multiple 9W devices are present, it
wastes enough power budget to deny power to other lower-power powered devices.
Since Cisco Discovery Protocol explicitly signals the actual power required,
the wasted power is returned to the PSE power budget.
Periodically, the PSE checks to see whether the powered device is still
present and requires power and also implements checks in order to detect
conditions, such as where a short circuit occured between transmit and receive
pairs. Cisco implements two mechanisms in order to detect these conditions. The
first is an extension of the pre-standard discovery protocol, whereby a
discovery signal is transmitted periodically. If the received discovery signal
has the same amplitude as the transmitted signal, the PSE removes power,
because there is a short circuit. If the PSE receives a discovery signal that
is attenuated by the low pass filter, the PSE maintains power to the powered
device. IEEE 802.3af-2003 is the second mechanism supported by Cisco. With this
mechanism, the power draw is monitored, and if it exceeds a specific value for
a specific time period, power delivery is shut down to the port.
What are the power requirements for the various models of the IP phone
CP-7906G (5W) (Class 2)
CP-7911G (5W) (Class 2)
CP-7941G (6.3W) (Class 2)
CP-7941G-GE (12.9W) (Class 3)
CP-7961G (6.3W) (Class 2)
CP-7961G-GE (12.9W) (Class 3)
CP-7970G (10.25W) (Class 3)
CP-7971-G-GE (15.4W) (Class 3)
CP-7985G (12.55W) (Class 0, Not full brightness)
IEEE 802.3af Device - Class 0 (15.4W)
IEEE 802.3af Device - Class 1 (4W)
IEEE 802.3af Device - Class 2 (7W)
IEEE 802.3af Device - Class 3 (15.4W)