Re: PoE, Comcast Modems, and Service Outages

[Aaron de Bruyn via NANOG <nanog () nanog org>]

Just to be clear Josh, I'm not insulting him.

I find the situation extremely difficult to believe based on my (possibly
incorrect) understanding of how PoE works and very (very!) basic knowledge
of things like RF interference—especially when it comes to Cable networks.

I mean, the call literally went like this:
"Thank you for calling Comcast this is <tech>, how can I help you?"
"Hey, can you remotely reboot the modem on account 12345? We're seeing high
packet loss and latency starting about 10 minutes ago."
"Yeah...uh...do you have a PoE switch at that location?"

When you hear hoof beats, look for horses, not zebras. As a first
troubleshooting step, I certainly wouldn't jump to "it's PoE". Granted, I
have no idea if Comcast has "PoE Buildup" graphs in their internal tools,
but based on my conversations with tons of other Comcast reps about tons of
other Comcast connections and never hearing one of them mention those
graphs, I'm leaning towards him lying through his teeth.

Lastly, the reboot of the Comcast modem "fixed" the issue.

I saw one of the IT guys from another office in the complex a few minutes
ago and he said their internet had problems at the same time. Comcast has
been out to the equipment room in the facility ~5 times over the last few
years to "adjust" things...so I'm still leaning towards this being
something more common like faulty equipment, bad signal levels, etc...and
not "It's because you have a PoE switch".

-A

On Tue Mar 29, 2022, 07:42 PM GMT, Josh Luthman
<josh () imaginenetworksllc com> wrote:

There's a certain manufacturer of TDD radio where the CPU clock is at the
same frequency as what Verizon's enodeB will transmit.  Even at miles away,
it can and will cause PIM issues.  Again, don't rule it out.

Maybe he's just looking for a simple answer that 99% of callers will accept
and it makes them happy.  When a customer of mine tells me they think it's
something and I know it's off, I just let them believe in their statement.
There's no reason to go after this tech and insult him, all that's doing is
making everyone miserable.

On Tue, Mar 29, 2022 at 3:26 PM Joe Greco <jgreco () ns sol net> wrote:

> On Tue, Mar 29, 2022 at 03:07:47PM -0400, Josh Luthman wrote:
> > 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.
>
> You can run cable lines next to A/C wiring and get problems too.  Or
> ethernet lines next to A/C wiring.  That does not justify wild claims
> about PoE such as what this tech was making, and until someone shows
> me a graph of "PoE buildups" observable via SNMP or whatever the
> cable company is using to graph trends, it seems pretty clear that
> this is a bogus answer.
>
> There's a lot of difference between "we observed this very specific kind
> of interference related to PoE in a particular circumstance" and the
> crazy generalizations being made by the tech.  Asking to please make sure
> your switch is grounded properly?  That'd be good.  Asking for PoE to be
> disabled on the port?  Yeah fine.  Suggesting separation of cables?
> Sure.  Checking for proper grounding of the ground block (on the cable
> inlet)?  Sure.  There's room for things to happen.
>
> I'm all for investigating with an open mind, but I draw the line at crazy.
>
> Given that so much of the world works on PoE, it seems like the other
> potential resolution would be to note that there's an implication here
> by the tech that Comcast's hardware is standards noncompliant and ask
> them what they plan to replace their cheap CPE with.
>
> ... 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