Hi @mike,
I did some testing today in a similar environment (but more controlled): two vlans going through a router.
I placed the iMac with the Roon Core on a separate VLAN by itself, and the rest of the home network stayed on the same VLAN.
This time I set up an IGMP proxy at the router, linking both VLANs, with the VLAN having the Roon core in the downstream.
With this configuration, the roon client/remote’s multicast packets make it to the VLAN with the Roon Core, thus the remote discovered the server and, furthermore, the server recognized the local player, and could play music on it.
With this setup, however, the roon core lost sight of the Airplay server in my receiver. So I went to fix that too.
The fix also required configuring the router, placing a mdns reflector on it, in charge of shipping mdns packets between the vlans. After this, the Roon core could detect the Airplay server, however it was not able to play music on it.
After trying several approaches, I decided to test if the iMac with the roon Core could ship audio to the Airplay server (without Roon). The result is that IT COULD ship audio to Airplay. So. I suspect that your implementation of Airplay must be lacking something (or making some sort of assumptions) about the discovery responses from the Airplay server…
I have seen the mDNS response packets from the Airplay server being received by the Roon Core machine, however Roon does not seem to make use of its contents in an appropriate way.
The only thing I can think of to explain this behavior is that Roon may be taking the IP address sending the mDNS response as the IP of the Airplay server, and this is not the case now, as those packets show the IP of the router, which is running the mDNS reflector. Of course, I could be totally wrong, as I did not have that much time to analyze the network traffic, and do not fully understand how Airplay works…
In sum, I think I resolved my original problem, but I discovered another one (the Airplay issue) that I do not know how to solve…
If there is interest I can give some details of my configurations, although they are dependant on the router I am using a ERLite from Ubiquiti)
Regards