Running Into Charging Issues with Your PoE+ to USB-C Converter or Other PoE Devices?
There’s a little known default feature in your Cisco enterprise switch that prevents you from getting the most out of your PoE+ devices, especially when using a PoE+ to USB-C converter for iPad Pro, Surface Go, Samsung Tab, and other devices.
While your Cisco 9400 or 9300 model switches are rated for PoE+ and capable of negotiating PoE+ class 4 devices, they will default to only negotiating 15.4 watts with your PoE converter unless you configure your switch for 2 event classification. USB-C devices, including the Surface Go and Surface Go 2, require 18 watts or more to charge properly and communicate. You can see why below
In that case, you’ll know the default setting is wrong because:
1 – You’ll see the port only negotiates 15.4 watts of power
2 – Your USB-C device will either not charge properly or will begin to lose battery charge even if it shows it is charging
Configuring Your Enterprise Switch for 2 Event Classification
Fortunately, it’s a quick, well-documented setting you just need to update for the ports you’re planning to use for your PoE+ device. You can find the instructions from Cisco here:
Or just go to your switch terminal and follow these steps:
Switch# configure terminal
Switch(config)# interface gigabitethernet2/0/1
You can also configure multiple ports on a switch at one time by choosing a range like
Switch(config-if)# power inline port 2-event
Why Doesn’t PoE Texas Just Make Their Devices LLDP Compatible?
Simply put, LLDP and USB are not compatible power standards. They simply work differently.
While we recognize the huge benefits to our customers to not require them to reconfigure their switches, the challenge comes down to what LLDP is and how USB works. LLDP is designed for computers and processors to share vital information with each other at the lowest level possible which works great for an end device like a computer or a phone with a processor on-board and its MAC address, in particular for our case one that can boot and start communicating in a low power mode.
Unfortunately, USB-C devices and the USB protocol doesn’t behave that way. Take a Microsoft Surface Go or an iPad Pro, for example. Their default initial power demands are 18-22 watt PD profiles. If you connect a USB-C hub or dock that offers to provide power to the device without the correct power delivery profiles sufficient to charge the battery, it will reject the device and disable the USB-C port which protects your device from defective or improperly built USB devices. You want that.
On the other hand, it is the end device like the PC or tablet that does LLDP communication with the network once it is connected. So it will not connect to the network through the USB-C port until it has received a correct power profile from the hub. So in this case, the iPad Pro or Surface Go cannot communicate its power requirements back to the network switch until after it has received sufficient power to make the connection.
And, since you’ll likely point out that the LLDP can happen before you connect the power by disabling the USB-C PD, yes, you can conceivably start with a charged tablet negotiating power then turning on the Power Delivery charging. There are two challenges there as well:
1 – We would need to customize the LLDP drivers on all devices which is considered basically a change to the kernal level programming since this communication works at the lowest levels. Device manufacturers control that very carefully and will be slow to consider that change.
2 – Even if we did achieve that level of change, if the device drains the battery for any number of reasons including power outages or being disconnected for a time, the tablet or PC would be unable to charge, boot up, connect LLDP, and start battery charging without the PD profile. Experience has shown customers cannot guarantee that only charged devices will connect to the network.
So, yes, making switch changes does drive an added layer of configuration, the change provides more reliable and consistent performance in all situations with your USB-C device.