GS108Ev3 firmware update triggering Orbi subnet reconfiguration

For anyone else who stumbles upon this thread -- reposting my solution (from the Orbi side) here as well.

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CrimpOn : Did some final testing and now confident in the conclusion.

What triggers the "reconfiguration" behavior appears to be whenever Orbi detects any other network device on the other side of it's Internet (WAN) port sharing the same subnet. It doesn't matter if there's an actual conflict or not -- simply it's existence. From a consumer support standpoint this is actually a pretty clever mechanism (though I wish they'd give us the option to disable it for various use cases).

In my *personal* case - the issue was that I'd put the ports in their respective VLANs (10 for WAN, 20 for LAN) but left VLAN 1 in place. Regular (untagged) traffic was working just fine. My guess is that during a firmware update on those GS108Ev3 switches, it was sending out a broadcast across all configured VLANs, Orbi was seeing that broadcast on the Internet port, hence reconfiguration is triggered. I removed VLAN 1 from all ports and haven't been able to replicate the problem since.

As for your question about the use case for the two switches? This as a method to extend out multiple networks over a single cable. My current configuration looks like this:

VLAN 10 = WAN Traffic
VLAN 20 = LAN Traffic

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BASEMENT

GS108Ev3 - Port 1: VLAN 10 Tagged, VLAN 20 Tagged

GS108Ev3 - Port 2: VLAN 10 Untagged
GS108Ev3 - Port 3-8: VLAN 20 Untagged

AT&T Gateway LAN plugged in to GS108Ev3 Port 2

UPSTAIRS OFFICE

GS108Ev3 - Port 1: VLAN 10 Tagged, VLAN 20 Tagged

GS108Ev3 - Port 2: VLAN 10 Untagged
GS108Ev3 - Port 3-8: VLAN 20 Untagged
Orbi Internet Port plugged in to GS108Ev3 Port 2
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Port 1 is the single physical cable running between the two switches. It will pass all traffic for either VLAN (LAN or WAN side) without either seeing one another as the traffic is "encapsulated" (isolated from each other). Port 2 on each side is where you plug in the WAN side of things. In the basement I have several runs from around the house plugged in to ports 3-8 (LAN). In the Office, I also have a bunch of devices plugged in to 3-8 (LAN). Two separate floors but they'll all end up in the LAN side.

The notion that someone shouldn't be using managed switches in a network topology is a silly one, assuming the configuration is correct. In my particular case (and the fix to the original problem I posted about) turns out to simply be "don't let Orbi's Internet port see any traffic with a subnet that matches it's LAN side". Simple enough, makes a lot of sense. Once I understood what was triggering the reconfigurating, finding the root cause was simple.

(Extra thanks to schumaku for the sentence that led to the conclusion -- "Somehow the Orbi system does see any 192.168.x.x network on it's WAN/Internet port.")