How much can you reduce POE watts on certain devices?

[Joe Lentine]

                   Hello all,

I'm getting currently installing a full Valcom IP sound and clock system , along with all new Cisco 2960 gig ps-l switches. All devices run off of POE. In the past, I have read that most AP's only draw approx. 12.5 watts (va's). But you must take into account length of wire to add your additional draw which puts you at the 14.5 they auto draw.  My question being that the clocks I have auto draw 15.4 watts, which is eating up my 370w of power on my switches. The spec sheet on the clocks show only a power consumption of 4.5 watts.  So today I turned a few ports down to 4.5 watts and everything still worked fine... Does anyone have any expierence by chance with Valcom devices or just turning the power down on devices in general? I can imagine that a clock is drawing enough to care about the length of the wire run...

any info would be great

Thanks,

Joe

[Leo Laohoo · Accepted solution]

The spec sheet on the clocks show only a power consumption of 4.5 watts.  So today I turned a few ports down to 4.5 watts and everything still worked fine... Does anyone have any expierence by chance with Valcom devices or just turning the power down on devices in general? I can imagine that a clock is drawing enough to care about the length of the wire run...

I have no experience(s) on the Valcom clocks but this is my guess of what's going on.

It's got nothing to do with the length of the wire.  I believe it all boils down to whether or not Valcom supports Cisco's CDP.  When a power-end-device is connected to a Cisco PoE port, the switchport will push full power (15.4w) and the switchport will wait for the PED to respond to the CDP packets sent by the switch to the PED.  If the switchport doesn't hear anything back from the PED then the switch will continuously pump 15.4w of power.

The CDP return packet (the switch will expect) will include a statement telling the switchport that "Hey you there, I can fully operate at BLAH watts of power".

That's the automatic method.  If the power-end-device doesn't support CDP then you can mannually set the ports to limit the amount of PoE you want, and optionally, WHEN you want it (via Time-based Access Control List) or via Cisco's EnergyWise.

[Joe Lentine]

Oh man.. you just struck that one perfect! I just remembered when I read the valcom config sheet that it said to turn off CDP on all those ports!!! thanks so much!!

Joe

[Leo Laohoo]

valcom config sheet that it said to turn off CDP on all those ports

Thanks for the ratings.

However, I don't understand the logic behind this.  Did the Valcom configuration guide said to turn off CDP if it's NOT connected to a Cisco switch?

I mean, if the Valcom supports CDP then why bother turning it off.  (Sorry, not asking you Joe.  I'm just asking aloud hoping somene can shed some light.)

[dominic.caron]

Just one precision, the switch does not pump 15.4 watts of power to the device, it reserve the power. If the device needs 5 watts of power to operate, it will take it but the switch still reserve 15.4w. There is also always some power lost in the cable du the resistance of the cooper wire. If you know the power needed by your device, add 1 watt to that number and configure the allocated power to that value.

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[Leo Laohoo]

the switch does not pump 15.4 watts of power to the device, it reserve the power. If the device needs 5 watts of power to operate, it will take it but the switch still reserve 15.4w.

No expert in how PoE negotiates but there has to be a way for a powered-end-device to tell the upstream PoE switch how much power the PED needs.

It's like this:

You have an unknown powered-end-device.  You (as a person and not a PoE switch) don't know how much power it requires.  You plug it in and is dead (for a few seconds, right?).   Then this PED lights up and the all the bells and whistles start firing up.

If you have a Cisco PoE switch and you regularly type in the command "sh power inline ", you'll see the description goes from a blank line to something like "IEEE" and the power goes from 0.0w to 15.4w.  After the awhile the power pushed by the switch "stabilize" to the appropriate level.   If you have Cisco PED, like phones or WAPs, you'll see the "IEEE" label change to an appliance name.  If it's a phone, it'll tell you EXACTLY what model that is.

However, with the advent of PoE+ and uPoE, I believe it's still the same, the switch will push full PoE power 14.something or 15.4w to the port and will "stabilize" after CDP are exchanged.

[Joe Lentine]

Leolohoo it said if your using Cisco to turn CDP disable on the switches. This is because valcom does not support anything with Cisco. Only Cisco proprietary devices can use CDP, also I have seen foundry use CDP but that is beside the point. And he sounds right about the reservation of the power. But turning off CDP will stop the reservation and allow the devices to only use and reserve what they need

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[Joe Lentine]

Well CDP is a fast protocol and with my experience it works right away.. But in my case the port is reserve the 15.4 because it is not receiving a CDP packet. So by turning it off, the PED still draws what it needs but when you do your sh power in, it will show the correct amount of power being drawn

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[Leo Laohoo]

Joe and Dominic,

I think this is what I was trying to imply:

Q. How do I determine if the IP phones that receive inline power use the Cisco pre-standard PoE version or the IEEE 802.3af standard?
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.

In regards to Valcom, I still can't get it through my thick skull as to the philosphy of turning off CDP when directly connected to a Cisco PoE switch.  I know you've provided the answer but I *know* it's me.