The ports are equal. No ‘IN’ and ‘OUT’.

Your ZP90 could however be showing its age. One of its ports may have gone faulty 

I had an ZP120:amp or whatever they called it.  A built in amplifier with a 4 port switch on the back.  Over time it got “sick” and I was told that it didn’t have enough RAM left in it after the numerous firmware upgrades and that if it was acting like a switch it could fail.  Only use it as an endpoint on a single ethernet feed.  it eventually died and was replaced by a new model that only had 2 ports on it (like this ZP90).  Does anyone recall this discussion?

Maybe as this is an older unit, its hit the same issue.  I was passing data through it.  Even when I removed the “output” line it was locking things up.  Right now I am trying to stabilize my network and then I’ll worry about re-introducing the SONOS box back into it.

Since all Sonos devices have only had 10/100 Mbps speeds on their ports, and not Gigabit ethernet speeds, I’ve never used a Sonos device as a switch/extender.  While those slower transfer speeds are perfectly acceptable for music transmission, they’re too slow for the rest of my network needs. 

The ‘OUT’ Ethernet ports on remote devices can be useful to provide wireless bridging for APs used for local control purposes, or even for normal phone internet connectivity. I get at least 10Mbps via SonosNet, with very low latency.

This is all wired, so no SONOS Net involved.  I try to keep anything not portable off of the wireless network.  SONOS responded above in the chain that there is no delineation between ports as IN or OUT.  Simply it has a 2 port switch built in.  As you stated, if you have a run somewhere and don’t want to add another wire, it is a good option.   

Post mortem on ATT: I got an upgraded modem, they removed 3 bridge taps on one of my lines, and it is running cleaner.  In hindsight, I think the ZP90 was causing most of the problems.  I also have a newer AMP:90 in the system.  After a couple of days, I’m going to re-introduce the ZP90 and see what happens.  I don’t know if I’m getting some type of deadly embrace between the other networking gear and the ZP90.  It was effecting LAn traffic as well as WAN traffic.  Nothing that is was actively involved in.

“AMP:90”? There’s a ZP90 (before it was re-badged as Connect), and there’s the Amp...

If you think wiring the ZP90 is causing problems it’s just possible that you have an STP issue. With multiple wired Sonos devices, and where their wireless (“WiFi”, though it’s really SonosNet) hasn’t been disabled in the controller, the wired path in between them has to forward STP traffic correctly in order to prevent loops. 

OK, I couldn’t find STP but I did find disable wireless (only possible if you are on a hardwire) which I am.

I’m not using it as a switch.  Just a dead end of the line device.  I’ll monitor over the next 24 hours and see what happens.  

I had to make these changes from the phone app. I couldn’t seem to be able to do it from the desktop app.  I’ll look again later when everything has settled down.