Switchport PoE behaviour

[gdg230001]

Hello,

Could someone just confirm some details about Cisco switch PoE configuration please? In particular on C2960X running 15.2

I have read the info on PoE at:

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960x/software/15-0_2_EX/int_hw_components/configuration_guide/b_int_152ex_2960-x_cg/b_int_152ex_2960-x_cg_chapter_0111.html#con_1909718

But I'm not 100% clear on the differences between setting  'static' and 'consumption' max values, is this summary correct?:

If I set a static value then the value I set is allocated to that port whether or not there is a device attached, and the port gets priority when PoE is allocated on switch start-up. I can also set a maximum value which, if breached, will result in power to that port being withdrawn. But this is only measured at the point of allocation and is according to the IEEE class of the device.

If I set a consumption value then if a device exceeds this value at any point then power is withdrawn (or if I set policing to 'log' then power is granted and the event is logged).

I think my confusion is made worse by the involvement of LLDP/CDP - assuming LLDP/CDP is not turned off then is it used whether either static or consumption values are set? So is it the case that in both static or consumption scenarios LLDP can be used by the device to negotiate power. It is only by switching LLDP off manually that negotiation with the switch is stopped?

Thanks for any help! It seems like this should be simple but somehow isn't!

[Leo Laohoo]

Exactly what are you trying to do?

[gdg230001]

Well mainly I just want to understand! The reason this comes up quite regularly in my work is that we run a wireless service and the APs often have little niggles in the way they negotiate power, if I can understand the switch behaviour then i can suggest workarounds if there are bugs that affect the APs that we send out.

[Leo Laohoo]

Are the APs Cisco-branded?

[gdg230001]

No, we use Aruba.

[Austin Sabio]

 There are three PoE modes: 

• Auto: accomplished via CDP protocol where the switch automatically senses PoE class and provide max power of that class within power budget = for PoE devices. 

Switch(config-if)# power inline auto

• Never: disable= No power detection + No power on the switch port =for Non-PoE devices.  

Switch(config-if)# power inline never

• Static: to pre-allocate power to the switchport.

Why static?

• Priority: to guarantee that most important PoE device receives power.

considering IEEE Power Classifications table below:

Class	Maximum Power Level Required from the Switch
0 (class status unknown)	15.4 W
1	4 W
2	7 W
3	15.4 W
4	30 W (For IEEE 802.3at Type 2 powered devices)

• Non-CDP IEEE PoE devices that require less than typical PoE power=15.4W defined as class-0 "class status unknown" ---like Siemens IP phone uses 5W.
• Non-CDP IEEE PoE devices that require less than typical PoE power=15.4W defined as class-3 ---like Avaya IP phone 2620SW uses 8W.

• The max max-wattage option for Auto + Static modes =to disallow high-power powered devices on a port.

Switch(config-if)# power inline auto max max-wattage 

Switch(config-if)# power inline static max max-wattage

•  power inline consumption default [wattage value] (global or per interface) it's to reclaim logical power for non-Cisco PoE devices. Per cisco,its recommended to enable power policing with this command. 

Switch(config)# power inline consumption default wattage value

• This command sets a power ceiling which is more than the device will draw, but less than maximum value of the PoE class. 
• This command is useful when deploying non-Cisco class-3 PoE devices. 

"By using the power inline consumption wattage interface configuration command or the power inline consumption default wattage global configuration command, you can override the default power requirement specified by the IEEE classification. The difference between what is mandated by the IEEE classification and what is actually needed by the device is reclaimed into the global power budget for use by additional devices. You can then extend the switch power budget and use it more effectively."

SideNotes: 

• show power inline provides information in regards to the power budget.

• Available power
• Used power
• Remaining 

• when all available power is allocated= remaining ports won't power PoE devices. 

• Per Cisco,

• You should carefully plan your switch power budget, enable the power monitoring feature, and make certain not to oversubscribe the power supply.
• When you manually configure the power budget, you must also consider the power loss over the cable between the switch and the powered device.

• as PoE devices consume different power, you can calculate how many PoE devices can be powered using http://tools.cisco.com/cpc/ (considering your switch max power output)

Source: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960xr/software/15-0_2_EX1/int_hw_components/configuration_guide/b_int_152ex1_2960-xr_cg/b_int_152ex1_2960-xr_cg_chapter_01000.html#con_1874624

Please rate, if helpful. 

Thank you.

[Austin Sabio]

"I think my confusion is made worse by the involvement of LLDP/CDP - assuming LLDP/CDP is not turned off then is it used whether either static or consumption values are set? So is it the case that in both static or consumption scenarios LLDP can be used by the device to negotiate power. It is only by switching LLDP off manually that negotiation with the switch is stopped?"

I hope this is a direct answer. 

• CDP with power consumption—The powered device notifies the switch of the amount of power it is consuming. The switch does not reply to the power-consumption messages. The switch can only supply power to or remove power from the PoE port.

• Cisco intelligent power management—The powered device and the switch negotiate through power-negotiation CDP messages for an agreed-upon power-consumption level. The negotiation allows a high-power Cisco powered device, which consumes more than 7 W, to operate at its highest power mode. The powered device first boots up in low-power mode, consumes less than 7 W, and negotiates to obtain enough power to operate in high-power mode. The device changes to high-power mode only when it receives confirmation from the switch.

  High-power devices can operate in low-power mode on switches that do not support power-negotiation CDP.

  Cisco intelligent power management is backward-compatible with CDP with power consumption; the switch responds according to the CDP message that it receives. CDP is not supported on third-party powered devices; therefore, the switch uses the IEEE classification to determine the power usage of the device.

• IEEE 802.3af—The major features of this standard are powered-device discovery, power administration, disconnect detection, and optional powered-device power classification. For more information, see the standard.

same source: 

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2960xr/software/15-0_2_EX1/int_hw_components/configuration_guide/b_int_152ex1_2960-xr_cg/b_int_152ex1_2960-xr_cg_chapter_01000.html#con_1874624

Please rate, if helpful. 

Thank you.