The number of RSTP experts on this forum can probably be counted on one hand.

(I am not one of them, though I do know about RTSP which is something else entirely).

do sonos people actually look in these boards?

Only community moderators, to my knowledge. I assume, having run similar boards, they collate data and pass it back to the Product Manager and Engineering manager to decide what issues need to be addressed, and which go on to the backlog to be addressed later.

Plus even if they did, they would say “we don’t officially support complex network topographies involving vlans”. Or something.

I’m no “expert” but I have used STP aware switches together with hard-wired Sonos devices successfully. 

As you’ve already know, the easiest solution is to change the global STP setting on all the switches, to use “classic STP path cost”. This would lower the path cost to match those that Sonos will consider (10 for wired 100Mbit, and 100-300 for wireless links depending on signal quality, IIRC).

If you don’t want to change the global configuration of the switch, you can specify path cost per port, this includes all the ports that Sonos devices are connected to, and the uplink ports between switches.

You need to set the path cost to something low (1-10) on all of these ports, which will allow the total path cost for the wired link between players to be lower than the wireless.

Example:

Your Sonos Beam, will have a wireless link to the Sonos AMP. The path cost on that one could be as low as 100. 

The wired link would calculate 10 on the first port from the BEAM, then it’s an dumb switch which doesn’t add any path cost, then reach the core switch (which you have configured to 20000). Then it will exit on the switch adding another 20000 potentially (I’m uncertain if it adds cost in both directions). Enter the GS310 which adds yet another 20000 and exit to the Sonos AMP which will add 20000 more, and then additional 10 for the switch port of the AMP. 

The wired path cost is as you can see, tremendously bigger than the wireless link, and you can’t affect the path cost of the wired link. 

In your setup, you could also consider making your core router to be STP root, by setting it’s bridge ID to be lower than that of Sonos (it is 8000h), something like 4096, but I’m uncertain if that is possible on Netgear. This is nog a huge deal if one of the Sonos devices is the root device. 

You should however designate one of the wired Sonos players to be root, and you should select the one which is in closest proximity with your wireless speakers to make sure that they are trying to connect wirelessly to the optimal wired player (I assume you are using SonosNet and not Wifi mode).

Path cost of ports that doesn’t house any Sonos gear (on any level, ports which connect to switches that house Sonos gear is affected), shouldn’t matter, as far as I know.

I hope this helps. 

It helps tremendously.  I really appreciate the help!