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PoE Negotiation/Reporting on Cisco Switches and Aruba APs

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  • 1.  PoE Negotiation/Reporting on Cisco Switches and Aruba APs

    Posted 19 days ago
    Hello!

    I apologize in advanced if this has already been addressed a few times.  I did a quick search but didn't find anything.  I am new to Aruba and running into an issue with AP-515s connected to my Cisco C9300 running IOS-XE 16.9.4.  According to Aruba Central, the power draw on my AP-515 is 10.77W.  However, running show power inline on the switch shows power usage at 30W.  

    To be clear, I think this is a Cisco issue, not an Aruba issue.  I have also experienced this issue on other non-Cisco APs (E.G. Ruckus) on Cisco 3850s running both 16.9.X as well as older versions of IOS-XE.  I think its an issue with Cisco not properly negotiating and/or reporting power usage with lldp clients.  I have spent 20-30 minutes digging into Cisco documentation, but to no avail.  My concern is that the switch is allocating the maximum wattage on a per-port basis, and that means we will not be able to power on every access point in a couple of closets.

    Just curious if any seasoned admins had run across this and had a quick answer.  Thank you in advance, and I am more than happy to provide additional details should they be required.


  • 2.  RE: PoE Negotiation/Reporting on Cisco Switches and Aruba APs

    Posted 15 days ago
    Hi procopius1980,

    Good spotting. This is commonly unnoticed until switch PoE budget is prematurely exhausted. 

    There are multiple values when it comes to PoE. There is total PSE (switch) budget, the maximum provided per device/port, the reservation for a particular device and the actual draw. Aruba Central will be indicating the actual power used by the AP. Based on your reading were you able to ascertain what you're seeing on the Cisco side is a report of power usage/reservation and not just the maximum potential power?

    In an Aruba switch there are multiple methods configurable to determine how much power to reserve for a device on a port. e.g. here on Central's documentation you can see that power allocation can be set to be done by usage or by class. 

    Also, LLDP MED can be used to query the powered device and discover the power requirement. This way the port doesn't need to send 30W when 11W is required. Both the switch and the AP will need to support the power management TLV for LLDP MED. The AP-515 supports this.

    It appears in your case the switch is pre-allocating the full possible power draw to the port. It could be possible to find a scenario where across the entire switch multiple ports were allocated only what is required but during a ramp up of requirements the PoE budget is exhausted. This could result in a failure of devices or some sort of loss of service. Check if the port is statically designated the max wattage or if it's set to the default 'auto'.

    The data sheet for the AP-515 states that it's worst case power consumption when powered using 802.3at/bt is 20.8W. It might worth trying to allocate this as a static value to the port. However, it might be the case that the switch detects the AP can consume more than 21W as per it's PoE class and decides not to provide any power at all. But it's worth a try!

    power inline static max 21000​

    Double check this is the correct command at the interface level. 

    Hopefully I've provided some context to try some things out. Good luck! Let us know if you find out the commands required.


  • 3.  RE: PoE Negotiation/Reporting on Cisco Switches and Aruba APs

    Posted 15 days ago

    Thanks!  Here's a rundown of what we've tried thus far.

    1). I have IPM enabled.  Currently only disabling USB, alt_ethernet, and turning down 2ghz_power by 3dB.

    2).  Next, we tried power inline auto max 15400.  AP would not boot, issuing show power inline showed power-deny.
    3).  Then tried power inline auto max 20800.  Same result.
    4).  Updated AP firmeware from 8.6.0.2. to 8.6.0.8.  I read in this thread that there was an issue in 8.6.0.4 with the AP requesting full Class 4 PoE upon bootup. https://www.reddit.com/r/networking/comments/haiwuc/aruba_poe_power_utilization/
    5).  After reboot, we tried power inline auto max 25000 and power inline static max 25000.  

    I have the AP back in Central and I am reviewing IPM and documentation to see if I missed a step.




  • 4.  RE: PoE Negotiation/Reporting on Cisco Switches and Aruba APs

    Posted 14 days ago
    I've hunted through the 8.6.0.x release notes but couldn't find anything in relation full Class 4 PoE requests. I'll keep hunting. 

    I may have made an error in suggesting static max of 21000 as I didn't account for cable loss. There are two numbers when it comes to PoE, there is the required power at the powered device and there is the supplied power from the power sourcing equipment. The PSE (or switch) needs to put more power into the cable as there is loss over the length of the cable run. If the AP may require 20.8W maximum then the switch will probably need to provide a bit more. 

    Cisco switches will not power the port if the device is requesting more than the maximum specified in configuration. 

    IPM is a good idea in my perspective, but it's not the answer here. IPM is about preserving AP function in the event the PSE doesn't provide enough power. This issue is more down to how the switch reserves power for the port.

    You're not alone here, I'm seeing this across a few threads. 

    Cisco's documentation states: 
    You can specify the maximum wattage that is allowed on the port. If the IEEE class maximum wattage of the powered device is greater than the configured maximum value, the device does not provide power to the port. If the device powers a powered device, but the powered device later requests through CDP messages more than the configured maximum value, the device removes power to the port. The power that was allocated to the powered device is reclaimed into the global power budget. If you do not specify a wattage, the device delivers the maximum value. Use the auto setting on any PoE port. The auto mode is the default setting.


  • 5.  RE: PoE Negotiation/Reporting on Cisco Switches and Aruba APs

    Posted 15 days ago
    Hi,

    Enable IPM in Aruba Central.

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    Tharindu Pamoda
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