PoE, Comcast Modems, and Service Outages

Josh Luthman josh at imaginenetworksllc.com
Tue Mar 29 19:07:47 UTC 2022

We've routinely seen where lines not even connected to the same circuit in
any way (ie an OTA antenna coax line and cat5 POE) cause issues with one
another.  As much as we would all love to have a perfect line in the sand,
there isn't.  Don't rule anything out until the issue is resolved.

As someone that sees this in the field and watches people simply hate on
someone because there's a frustrating situation, it's worth taking a breath
before too upset.

On Tue, Mar 29, 2022 at 2:50 PM Joe Greco <jgreco at ns.sol.net> wrote:

> On Tue, Mar 29, 2022 at 11:21:28AM -0700, Aaron de Bruyn via NANOG wrote:
> > I just got off the phone with a Comcast tech, and wanted to double-check
> my
> > sanity.
> >
> > 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
> > your modem".
> >
> > 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
> > standard.
> That's ridiculous, as you already know.
> Their crappy equipment needing rebooting every few weeks, not ridiculous.
> Their purchasing gear from incompetent vendors who cannot be standards
> compliant for PoE PD negotiation, tragically plausible.
> Ethernet electronic differential signalling not being handled properly
> with respect to grounding or other issues, not unheard-of.
> Ghosts of PoE floating around a network through other devices, causing
> weird problems on the far side of properly installed and standards
> compliant
> gear, ah, super-unlikely, I'll go so far as to say nah, but in this
> audience
> I am positive someone has a counterexample SOMEWHERE, cuz, you know, the
> world's a strange place and there's broken stuff out there.
> He's got graphs showing it every 24 hours?  Liar, liar, pants on fire,
> lazy SOB is looking for an excuse to clear you off the line.  Where the
> heck does this "24 hour" cycle even come from?  What SNMP OID is there
> for "ghostly PoE build-up"?  What crontab is there that would clear out
> such buildups in the router's daily run?  What capacitor would store up
> juice for precisely 24 hours?  What's the mechanism here?  CURIOUS MINDS
> Been doing PoE everywhere for years and this is the stupidest thing I've
> heard this year so far in the networking category.
> Next time, play dumb.  People who can, do.  People who can't, tech support.
> Worth remembering.
> ... JG
> --
> Joe Greco - sol.net Network Services - Milwaukee, WI - http://www.sol.net
> "The strain of anti-intellectualism has been a constant thread winding its
> way
> through our political and cultural life, nurtured by the false notion that
> democracy means that 'my ignorance is just as good as your
> knowledge.'"-Asimov
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