I'm sharing this in case anyone comes here with a similar problem (although it's possible this has been discussed here before).
For the past couple of weeks I've been trying to troubleshoot why my hAPax3 is only connecting at ether1 to the Spectrum cable modem at 100mbps. Both modem and ax3 has 2.5gbps ports.
I did the usual troubleshooting: Replaced the cable with a known good one, tried a different ax3 port, reset and restarted and combed every config line.
I contacted Spectrum and they sent a tech -- twice. Each time he could not distinguish between "internet speed" and ethernet connection speed and refused to change the modem.
I was about to try a third time when I noticed log entries that I have had in place a long time that alert me to brown out conditions. It seems my house regularly drops before 110 VAC.
Over the holiday weekend the entire area had a black out (no power). When it came back up (36 hours later), the voltage was steadily in the 117-120 range and the ax3 was connected to the modem at 2.5gbps.
I also had on my list to bring a laptop to connect directly to the modem and to swap the ax3 with another device, but hadn't gotten to it yet.
I hope the utility company's repairs actually fixed the low voltage condition, and the cause of the 100mbps connection was the low voltage condition. I'm monitoring line voltage with the ROS script and UPS.
Modern AC/DC adapters are typically auto-adatpting 110-240V on the AC side.
I doubt that anything above (say) 95 or 100V[1] would change anything on the DC side, and at least the Ax3 has also such a wide input range, 12-28 V that even if the theoretical 24V from the adapter drop a little it won't change anything.
[1] very likely there is a limit voltage, let's say 80 V, under which the converter simply doesn't provide any voltage on the DC side.
I can assure you that with my refrigerator disconnected from mains and all TV's disconnected from both mains and antenna (and also with all of them connected) the ethernet 2.5 Gb link between switch and router remains at 2.5 Gb and does not negotiate down to 100 Mbps. (which was the issue the OP reported).
Normally the power supplies sold today can accept anything between ~100 (maybe 90) and ~250 volts. When you have a failure of the neutral conductor in your house, the voltage can drop very low on some outlets (and increase on others), but it will normally not cause permanent damage.
In Europe, we generally have 3-phase power of 3x230/400V which means a normal outlet has 230V. However, when the neutral conductor gets interrupted somehow, the actual voltage on an outlet can vary from very low up to 400V and that certainly causes problems and permanent damage.
Of course there are houses that do not have 3-phase installations, but the NETWORK is 3-phase so the problem when the neutral is interrupted remains the same. The street will normally have 3-phase feed and alternating phases used in adjacent houses.
Or do you know of European countries where it isn't like that?
From reading the post it seemed like three phases arrived to homes, but that Is common only in parts of Europe (north/north-east) and uncommon in other parts (south/west) , loosely:
The distribution Is three phases almost everywhere, yes.
Very likely in the US losing the neutral is not as bad as it can be in Europe, as the max voltage you can have is 240 voilts or so, so everything that uses "tolerant" power supplies won't likely fry. Refrigerator, washing machines, etc. are anyway subject to the same issues.
As seen in the discussion on that link, the situation is not red/green. Over here every house has a 3-phase feed but usually a 1-phase meter is installed and everything runs off a single phase, but when you have 3-phase requirement you can just call the electricity company to have a 3-phase meter installed and there would be no digging or re-wiring required. This is often done by people who want to charge an electric car at home.
Here in Italy it greatly depends, in single homes you can usually have three phases (though it is not common and it costs quite a bit more), at the most they will add wires.
AND you'd better have a good (balanced) electric plant, as they will charge you for reactive energy at a very high price.
I have seen grown up people crying when the first biills arrived.
In apartments in multi-apartment complex it is not so easy, while surely the three phases arrive to the building they are dimensioned for giving a single phase to each apartment, there is no way they can give you permanently three phases without changing cables/wiring.
Often they will provide temporarily (as an example if you are re-structuring an apartment) the three phases for the duration of the works.
But Italy is - I believe - an exception on the lower side of the range of available energy, the typical contract (for an apartment) is 3 Kw, 4.5 kW is becoming more common as it is just enough to drive an air conditioner or two, 6 kW is rare, anything above 6 kW is extremely rare.
In Germany (to make another example) 15-20 kW is rather common, and is often three phases.
