Perpetual POE

You may not be aware, but the Perpetual POE feature allows a switch to continue to power devices connected to the switch, during a reboot. This is to ensure your network is back online as soon as possible after a rebook, as all your end devices are still up.

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What is Perpetual POE

Perpetual PoE; the ability to reload your humble Catalyst 9200/9300/9400 switches while maintaining uninterrupted PoE power. The result is that Powered Devices (PD) connected to the switch stay powered-on even during a switch reload.

Another part of this capability is Fast PoE; When AC power is restored in the event of a complete power outage, the switch port remembers the last power drawn (stored in NVRAM) and restores PoE to that port within 15 to 20 seconds.

It can bypass the traditional LLDP-based power negotiation meaning it does not need to wait for the IOS to boot up before enabling PoE on the port.

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Perpetual POE Use Case

I’d like to share an interesting experience I had with customer recently.

As part of their network modernisation program, they needed to understand and leverage all the High Availability, resiliency, and redundancy capabilities available to them in order to meet their business requirements centred around minimising any disruption to service as a result of either “Planned Events” or “Unplanned Events”.

The network solution was built around Cisco Catalyst 9000 switching platforms, which luckily for our customer, has many resiliency and redundancy features built into the hardware that they were not aware of.

A lot of you may not realise the extend of what’s available from the Cat 9K family when it comes to availability and resiliency, and that’s why I’m sharing just one these today, which I think is really cool.

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Enabling Perpetual POE

Worth noting that Perpetual PoE and Fast PoE is not enabled by default, it needs to be specifically enabled on a per-port basis using the following interface-level CLI commands:

• power inline port perpetual-poe-ha
• power inline port poe-ha

Something else to note is that Perpetual PoE and Fast PoE does not kick in just by entering these commands.

You will need to manually “shut” and “no shut” the port if the PD is already connected as the system needs to first learn the power draw of the powered device; and its good practice to give it 10 seconds before you “no shut” 😉

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Perpetual POE: Summary

Perpetual POE is not a well-known feature but can be very useful in environments where minimum downtime is required - for example hospitals.

The combination of Perpetual PoE and Fast PoE becomes really important for business continuity as we begin to see much more reliance on PoE powered “smart buildings” that use critical endpoints powered by PoE such as medical devices, lights, IoT sensors, badge readers and so on.

A key use case might be that the phones in a site are kept up and running through a switch power-cycle, so the phones are back online much quicker after a switch reboot. 

Hopefully this blog is of use for you if your environment requires minimum downtime - always something new to learn!

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