PoE, Comcast Modems, and Service Outages
Aaron de Bruyn
aaron at heyaaron.com
Tue Mar 29 18:21:28 UTC 2022
I just got off the phone with a Comcast tech, and wanted to double-check my
Somehow in the last 6 months I've managed to reach the exact same rep twice
when dealing with an outage or a degraded service event.
I asked him to remotely reboot the modem because there was high packet loss.
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.
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.
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.
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.
Again, I double-checked the port and said "It's not sending PoE to my
router, but even if I were, I highly doubt PoE would leak through a PCI
card to the opposite side of the chassis to the on-board NIC and out to
He insisted it happened "all the time" and he had previously fried
equipment by plugging it into a PoE switch. He insisted that he's also
handled quite a few calls relating to this magic PoE problem over the years
and Comcast has internal tools that show graphs of how much PoE power
"builds up" inside their modems and he "can see a buildup in my router that
resets every 24 hours".
I didn't have the heart to tell him that I manage about 40 networks that
have Comcast connections...and they *all* have identical FreeBSD boxes
acting as their router, and they are *all* using the exact same PoE
switches at every location with all ports set to PoE+...and we only have
degraded service or outages after ~30 days at ~3 locations.
Slightly off-topic, but if I call Comcast about outages or degraded service
and any *other* tech but this guy answers, they all say "you need to unplug
your Comcast modem and plug it back in once every 3-4 weeks" and they act
like it's normal to reboot the modems every few weeks. In fact, last week I
wanted Comcast to check on a modem setting at one location and they said
the modem had been up for over 127 days and it should be rebooted. I said
"it's up and working fine, why would I reboot it?".
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
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the NANOG