PoE

NightShade03 · December 2009

Can someone explain the power inline command a little better? Specifically its use with non-cisco ip phones. It's my understand that if you plug a non-cisco phone into a PoE swtichport the switch can't negotiate a power level for the phone and defaults to 15.4 W for the device (which can cut the number of devices in use in half on a 48 port switch).

Someone correct me if I'm wrong.

jason_lunde · December 2009

From: Power over Ethernet (PoE) Power Requirements FAQ [Cisco Unified IP Phones 7900 Series] - Cisco Systems

"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. "

Ya, since the phones are not Cisco, they dont support CDP, which helps them negotiate a power (mine go to 12W). If you have multiple poe devices all using 15.4w, then the power will soon max out on that switch or switches, and you will not be able to run any more poe devices. You can view your available and consumed power with the command, show power inline. Hope this helps.

"It's my understand that if you plug a non-cisco phone into a PoE swtichport the switch can't negotiate a power level for the phone and defaults to 15.4 W for the device." This is partially true, but based on the IEEE classification of the device: Here is what I could find:

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)

So if a device was a class 3 IEEE device that COULD run off of 9w, but was not a cisco device that could not use CDP to negotiate the 9w, the switch would have to allocate the entire 15.4w max to that port...therefore allocating more power than actually required for the non-cisco phone....thats my understanding.

APA · December 2009

I haven't had to use the command before... but I'm sure you can override the IEEE PoE Classes statically via the following

power inline consumption (value)

From there you can configure the total power consumption to be used and also the wattage on a per device basis.

The command is supported at a global and interface level. This then allows you to prevent the Class 0 15.4W class being applied to devices that don't need that amount of power.

NightShade03 · December 2009

The global config was the key thing I was looking for. We are debating buying with a full blown Cisco phone system or Shoretel and one of Cisco's arguments is that Shoretel phones will suck all the power out of the switches....since Cisco owns much of the networking world I can't see how their statement was true. Knowing that I can set a lower default power level makes the Shoretel solution more viable.

PoE

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