Cisco AP not powered by 802.3at from PoE switch

One of the requirements when introducing a wireless access point (AP) is “power supply”, isn’t it?
There are many factors that must be considered, such as the amount of power supplied to the AP and the power supply method. If they are not met, the wireless transmission power will be insufficient and the cell range will be reduced, or the usable frequency band will be limited. There are various effects such as being done.

So I connected the Cisco Catalyst 9120 Series to a PoE switch that could power at 802.3at (PoE +) in my previous environment, but for some reason it didn’t power at 802.3at. I will write about this including the cause element.

Wireless AP is not powered at 802.3at.


I connected the AP (Catalyst 9120) to a switch (Catalyst 9200) that can supply power to PoE, but there was a phenomenon that power was not supplied at 802.3at (PoE +). Since the model of Catalyst 9200 has a model number that can be powered by PoE +, it is possible to secure a budget of up to 30 W with one port. Therefore, you can receive a maximum of 25.5W of power to the AP side by connecting it.

However, from the WLC screen, the AP seemed to be powered by only 15.4W, and the performance was not good.
* Catalyst 9120 requires power supply at 802.3at (PoE +) when using the full function.

I looked at various settings

I saw the PoE power supply limit on the switch side, but there is no particular setting such as “power in line”, and the PoE setting is also the default. PoE was not set on the AP side, and a large amount of the following was output in the debug / system message.

LLDP PoE negotiation FAILED !!

I thought it was very suspicious and tried various things.

The cause is CDP / LLDP

After all, the output of the system message seems to be a problem, and it was caused by the CDP / LLDP being disabled on the switch side.

no cdp run
no lldp transmit

When a Cisco AP needs power supply with standard 802.3at or higher, it needs to be negotiated by CDP / LLDP in order to exchange the required power supply amount and IEEE power class. If LLDP / CDP is disabled, negotiation will fail and power will be supplied with the default allocation of 15.4W.

Therefore, you need to enable either CDP or LLDP to ensure proper power negotiation with the connected AP, so be careful if you need more power than 802.3af (PoE)!

By the way, you can check if CDP is valid on the switch side with the following command.

switch#show cdp
Global CDP information:
        Sending CDP packets every 60 seconds
        Sending a holdtime value of 180 seconds
        Sending CDPv2 advertisements is  enabled