SOLUTION:

Following a tip in tp-link's support article on this issue

https://www.tp-link.com/us/support/faq/1794/ 

specifically, this part:

'A: Deco switching from Wi-Fi backhaul to Ethernet backhaul may create a short loop in the network for 1~2s. If your switch supports loop detection, it may block the port used by Deco. Please turn off the loop detection function on your switch, then reboot your switch and try again.'

I started tinkering with port settings on my Linksys switch.  It does not have a Loopback Detection feature, but I learnt that its function overlaps somewhat with the Spanning Tree Protocol (STP), which is a per-port setting.

So all I needed was to

1. Disable STP on the port to which a Deco Satellite is connected.

2. Wait 10-20 seconds

3. The Deco App will show that it lost your respective Deco Satellite unit.

4. Wait 10-20 seconds

5. The Deco Satellite is back online this time connected via ethernet.

This does trigger someting on the switch side, as the port STP status changes to 'Discarding' but it resolved itself after the page refresh back to 'Designated'.

6. I did re-enable STP on that port just in case - no issues so far, I guess the loopback event only takes place only once at switchover, as the support article suggests.

I did this on both ports/Sat Decos and it worked just fine - and there is no need to mess with the settings for the port that connects to your Main Deco.

Hope this helps you save a few bucks on a new switch (sorry, tp-link ;)

Som as in most cases, another RTFM situation :)

Cheers

PROBLEM as originally posted:

Hi All,

The system is set up as follows:

Modem > Main Deco in Router Mode > Managed Switch > 2 Satellite Decos connected to the switch each using its own wire.

I get no ethernet backhaul, as the satellite decos are shown as connected via Wi-Fi in the app.  I have excluded the wiring fault as Ethernet backhaul is confirmed as working in the app if I connect either of the Satellite Decos to the second port on the Main Deco directly using the very same ethernet cables.

However, when I plug the Main Deco and those ethernet cables runinng to Satellite Decos into a managed Linksys switch (lights blinking green on the switch as normal), I do not get the ethernet backhaul, with the Deco app showing a wifi connection between the Main and the Satellite units.

Many other devices plugged into the switch have the Internet, talk to each other, etc, so the switch is just fine.

Nothing fancy on the switch on those ports, no LAGs no VLANs.

FWIW, the Guest Netwrok is also working fine, with Internet access provided without any VLANs.

Rebooting did not help.  Swapping ports (From Modem/To Switch) on the Main Deco did not help.

What am I doing wrong?

Thanks