Perpetual and Fast PoE are friends, m’kay?

PoE icon was borrowed from LightwareHave you ever heard about two new PoE enhancements that are available on some Catalyst 3850 and all Catalyst 9K platforms? If not, you’re not alone. I only found out about these few months back by accident when I was troubleshooting some issues. There are two great features available for you to configure on PoE-enabled switches:

  • Perpetual PoE: preserves power during warm reload
  • Fast PoE: provides power to the port after cold start at pre-outage levels

The first one, once enabled, instructs the switch port to keep power uninterrupted during warm reload. For example, when you reload a switch or switch stack, your IP phones and access points will stay up. In some scenarios, such as FlexConnect AP, you will save around 5 minutes of downtime – FlexConnect APs do not reboot when WLC becomes unreachable. They stay up and keep existing clients connected. They are not forced to reboot. As soon as switch goes online, APs will re-join WLC and will begin accepting new clients. Great! Isn’t it? It won’t help to APs in local mode though as loss of WLC forces them to reboot even if they stay powered.

Second feature instructs switch to remember PoE power levels for every port (i.e. PoE config snapshot). If switch is turned off and then on (maintenance), or if there’s a power issue (cold start), then PoE will be supplied to all ports according to the last state in the first 10 seconds once switch receives power. Effectively, all your PoE kit can start booting without waiting for switch to boot and apply configuration first. Again, it will save 5 or more minutes of total downtime for PoE-based services!

Configuration is applied per port and depends on the IOS-XE version. Here how you do it on IOS-XE 16.6.x

interface interface-name-id
 power inline port perpetual-poe-ha
 power inline port poe-ha

Both features can be configured independently from each other.

I hope this was useful.

PoE icon was borrowed from Lightware

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