Question about PoE Usage

[alessandros1]

Hi all,

something is unclear, to me, about PoE usage in switches, specifically about Consumption vs. Budgeted power. For example, i have an MS120-8PF switch, which provides up to 124W with 30W MAX per port where i connect  four 802.3at Cisco APs, the switch will budget 30W per port, so total budgeted power is 120W, but  let's assume real consumption will be 24W per AP, so consumption is 96W in total. Now what happens if i connect another AP of the same type?:

1. i'll be able to power it as the real consumption will be 24W x 5 = 120W that is less of 124W provided by the switch?
2. i'll not be able to power it as we are consuming 96W and we have only 28W available, that is less theen 30W that will be budgeted for the port?
3. i'll not be able to power it as the switch budgeted 120W and we have only 4W available?

Thanks to anyone that will clarify!

Kind Regards

[RWelch]

Meraki switches will budget based on the PoE device classification, the budget is allowed to exceed available power as it's used to gauge overall power that might be consumed on the switch. Devices will continue to be powered until total power consumption goes over the available amount of power.

PoE Support on MS Switches

[alessandros1]

Hi RWelch

Thanks for your reply, this is something i read in the documentation, but my doubt is the following: as per i know, the switch will use CDP or LLDP to negotiate the AP power consumption but, if i'm consuming (not budgeting) 96W yet, and i connect a new AP, i think the switch will provide the maximum admitted power (30W) to the port, until CDP negotiate the power consumption, so the total consumption will be over the 124W provided by the switch. If so, the switch will still be able to power on the device?

[RWelch]

Devices will continue to be powered until total power consumption goes over the available amount of power.

[alessandros1]

thanks again for the reply, so in the end i will be able to connect all 5 the APs as the total consumption will be 24W x 5 = 120W that is less than the switch capability, correct?

but what if i in addition to the four cisco APs, i will connect a PoE+ device that is not CDP capable?

[Mloraditch]

https://documentation.meraki.com/MR/Other_Topics/Low_Power_Mode_on_Cisco_Switches

See the info on the standards and what power is provided when a PoE device does not support LLDP.

The document @RWelch previously is linked here: https://documentation.meraki.com/MS/Other_Topics/Troubleshooting_PoE_on_MS_switches
and in my reading of these three together, it seems the switch will just provide what is listed in the low power article (15.4watts)

[cmr]

Remember that Catalyst switches limit the power to the budget, so if the budget gets to the limit, no more devices will be powered up.

Therefore if you have APs that budget 25W, but only use 12W you can run 10 on an MS switch with a 125W PoE budget (though I wouldn't recommend running that close to the limit), but you could only run 5 on a Catalyst based switch.

[PhilipDAth]

124W - 96W = 28W available in budget.  You need 30W to power up.

I would expect the AP would not be powered up (the switch would leave the port PoE blocked)..

[alessandros1]

Thanks to all for the replies.

• Briefly, what i can understand is: Catalyst switches, when powering devices, takes "budget" in account, instead to consider "consumption"  as MS switches do;
• referring to my first example, If i use a CDP/LLDP capable AP i'll be able to connect it to the switch, as the total consumption will be less then the switch capability;
• still referring to my first example, if i use a non-capable CDP/LLDP AP, i'll not be able to connect as the switch will not be able to adjust the power to the real consumption, tries to put 30W but the switch can draw only 28W.

Probably i will test if i can in the future