PoE, Comcast Modems, and Service Outages

Brie bruns at 2mbit.com
Tue Mar 29 19:17:21 UTC 2022

On 3/29/22 12:21 PM, Aaron de Bruyn via NANOG wrote:
> Both times I've talked with him, he noted the high packet loss, started 
> to reboot the modem, and then asked me point-blank if we had any PoE 
> switches on our network.

This sounds like a guy who has created his own script for 'improving' 
his call resolution stats.  Lucky you!

> When I said "yes", he said I needed to disable PoE because it messes 
> with the Comcast modems and he can see "buildups" in his graphs that 
> show power is "leaking" to the Comcast modem every 24 hours.

I just choked on my ice tea reading that.

> For reference, our setup is:
> Internal Network ←→ PoE Switch ←→ My Router (FreeBSD Box) ←→ Comcast Modem
> I told him the Comcast modem isn't plugged into the PoE Switch, it's 
> plugged into My Router (FreeBSD box) and My Router does not negotiate 
> PoE+ and the switch shows PoE isn't being send to My Router's LAN port. 
> While the switch is capable of outputting old-school 24v PoE, it must be 
> specifically turned on for a port, and it's not enabled or used anywhere 
> on the networks I manage.


Yeah, you know when 24v passive POE is turned on because it kills the 
port on the other end that aren't designed to handle it.  Your router 
would likely have a dead eth port on it.

> When provided with that information, the Comcast tech still insisted 
> that the switch was sending PoE to My Router and it was "leaking 
> through" to the Comcast modem and that's why every 4-6 weeks the Comcast 
> modem needs to be reset. The tech insisted that switches that 
> /are/PoE-capable /always/send PoE even if the device doesn't request it 
> or negotiate it. Attempts to explain the difference between the old 
> 24-volt PoE and PoE+/++ were met with arguing that he's been in the 
> industry for decades and I don't know what I'm talking about...and that 
> all my problems would go away if I just disabled PoE everywhere on the 
> switch.

Last time I checked, ethernet ports require magnetics for normal 
operation, so they're isolated anyway.

And, you are right on the POE negotiation - there is no POE until 
certain conditions are met on the line to signal to the switch that the 
device requires POE.

Yes, in theory there could be interference if POE was enabled, it's not 
likely these days, and there's no interference from POE if its not been 

> Anyways, am I insane for thinking the tech was flat-out wrong? I 
> mean...occasionally some really bizarre stuff happens in IT...but this 
> seems extremely far-fetched and contrary to everything I know about the 
> PoE standard.

Nope, not insane.  Tech is full of it and really needs to be retrained 
properly by a supervisor.  I'd be asking for said supervisor next time 
you call in and get him.

Brielle Bruns
The Summit Open Source Development Group
http://www.sosdg.org    /     http://www.ahbl.org

More information about the NANOG mailing list