7940 and 802.3af

Answered Question
Jun 27th, 2007

Does 7940 support the 802.3af? I know that 7940 supports PoE. But what about 802.3af? Aren't they pratically the same?

I have this problem too.
0 votes
Correct Answer by mchandak about 9 years 3 months ago

Nopes, it does not support 802.af. 7940 IP Phone only supports Cisco Proprietary Inline Power.

Though if you are using a Cisco switch that supports 802.af, it would even support Cisco Prop Inline power supply

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Correct Answer
mchandak Wed, 06/27/2007 - 11:55

Nopes, it does not support 802.af. 7940 IP Phone only supports Cisco Proprietary Inline Power.

Though if you are using a Cisco switch that supports 802.af, it would even support Cisco Prop Inline power supply

rob.huffman Wed, 06/27/2007 - 12:47

Hi Rafael,

I just wanted to send a note to add to Mahesh's great info that describes the differences between the older Cisco Inline Power (proprietary) and the Standards based 802.3af;

Cisco Inline Power and IEEE 802.3af

Cisco launched Cisco Inline Power in March 2000 and has shipped more than 16 million inline power capable ports on the Catalyst 3500, 4500 and 6500 families of Ethernet switches. This innovation was quickly recognized within the industry and the IEEE started work to standardize Power over Ethernet implementations such that multi-vendor interoperability was enabled. Since the inception of Cisco prestandard Power over Ethernet capable IP phones, several powered devices have now been developed including Color IP phones, Video camera's etc. that utilize Power over Ethernet. With the ratification of IEEE802.3af, as with other Cisco innovations, Cisco will support both IEEE 802.3af and prestandard Power over Ethernet concurrently. Cisco has also extended prestandard power management extensions using Cisco Discovery Protocol negotiation to Cisco IEEE 802.3af compliant devices to further optimize PSE power management.

Applying Power over Ethernet requires that the device type be resolved to ensure that power is not applied to nonpower capable devices. To prevent unfortunate mishaps and to reduce the burden of network administration, Cisco and the IEEE devised mechanisms whereby the switch is able to determine whether a powered device or a nonpowered device is attached to a port. However, the phone detection mechanisms used by the Cisco prestandard Power over Ethernet implementation and IEEE802.3af are different in that the Cisco prestandard Power over Ethernet implementation uses AC powered device detection and IEEE 802.3af uses DC powered device detection. DC detection differs from AC detection in that AC detection transmits a low frequency AC signal and expects the same signal to be received back on the receive pair. DC detect applies a DC Current and detects the presence of a powered device by measuring the load applied by the powered device. It should be noted that Cisco IEEE 802.3af compliant devices support prestandard and IEE 802.3af detection mechanisms.

Using a Cisco inline power capable switch or Power Sourcing Equipment (PSE), the switch port will send discovery signals on active and inactive Ethernet ports to detect whether a powered device is present or not. It should be remembered that the powered device will not be powered at this time and therefore cannot bring the link up. It is therefore necessary to transmit the discovery signal on a continuous basis as a powered device may be plugged into the port at anytime.

Within a Cisco prestandard powered device, a low pass filter that is connected between the powered devices receive and transmit pairs allows the low frequency discovery signal to loop back to the PSE. A low pass filter is used as it allows the phone discovery signal to loop back to the PSE, but prevents 10/100 or 1000Mbps frames from passing between the receive and transmit pairs. Once the PSE detects that a powered device is attached to the port, the Cisco PSE will apply power to the port.

By contrast, the IEEE 802.3af-2003 standard uses a different powered device detection technique that uses DC detection to determine whether a powered device is attached and to which power classification the device belongs. An IEEE 802.3af-2003 PSE achieves this by applying a DC voltage between the transmit and receive pairs and measuring either the received current (Amps) or voltage (V) received.

From this excellent White Paper;


This is something to keep in mind when dealing with a variety of Phone/Switch models.

Hope this helps!


twissam Fri, 06/29/2007 - 11:36

Very intresting article, just to let you know that i had a case where i have 3750 PEO stacked switches, on some ports i had to sh and then no sh in order the phones to get power. very strange behavior, i did not looked further but i donno if someone have faced this before.

IP phones are 7941G-GE


EBEISW01# sho ver

Cisco IOS Software, C3750 Software (C3750-IPBASE-M), Version 12.2(25)SEB4, RELEASE SOFTWARE (fc1)

Copyright (c) 1986-2005 by Cisco Systems, Inc.

Compiled Tue 30-Aug-05 15:47 by yenanh

ROM: Bootstrap program is C3750 boot loader

BOOTLDR: C3750 Boot Loader (C3750-HBOOT-M) Version 12.2(25r)SEE4, RELEASE SOFTWARE (fc1)

EBEISW01 uptime is 11 hours, 4 minutes

System returned to ROM by power-on

System image file is "flash:c3750-ipbase-mz.122-25.SEB4/c3750-ipbase-mz.122-25.SEB4.bin"

cisco WS-C3750G-24PS (PowerPC405) processor (revision F0) with 118784K/12280K bytes of memory.

Processor board ID FOC1116Y1F3

Last reset from power-on

4 Virtual Ethernet interfaces

212 Gigabit Ethernet interfaces

The password-recovery mechanism is enabled.

512K bytes of flash-simulated non-volatile configuration memory.

Base ethernet MAC Address : 00:1B:90:2B:A6:80

Motherboard assembly number : 73-10217-07

Power supply part number : 341-0108-03

Motherboard serial number : FOC11150F6T

Power supply serial number : DCA1113A1M9

Model revision number : F0

Motherboard revision number : B0

Model number : WS-C3750G-24PS-S

System serial number : FOC1116Y1F3

Top Assembly Part Number : 800-26855-01

Top Assembly Revision Number : C0

Version ID : V05

CLEI Code Number : CNMWR00ARC

Hardware Board Revision Number : 0x09

Switch Ports Model SW Version SW Image

------ ----- ----- ---------- ----------

* 1 28 WS-C3750G-24PS 12.2(25)SEB4 C3750-IPBASE-M

2 28 WS-C3750G-24PS 12.2(25)SEB4 C3750-IPBASE-M

3 28 WS-C3750G-24PS 12.2(25)SEB4 C3750-IPBASE-M

4 28 WS-C3750G-24PS 12.2(25)SEB4 C3750-IPBASE-M

5 24 WS-C3750G-24T 12.2(25)SEB4 C3750-IPBASE-M

6 24 WS-C3750G-24T 12.2(25)SEB4 C3750-IPBASE-M

7 28 WS-C3750G-24PS 12.2(25)SEB4 C3750-IPBASE-M

8 24 WS-C3750G-24T 12.2(25)SEB4 C3750-IPBASE-M


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