I have to agree! My home network, as well as those I support use VLAN's as all host VOIP (Cisco / Asterisk) phones, as well as VPN's into 3 customer networks. No way this is happening at home without Ubiquiti, which support QOS (quality of service) and COS (class of service) as well as Multicast routing. I happen to have my audio segmented on a VLAN separate from all others (my wife does an intense nightly 20 minute Facebook/Instagram session for her business, I'm not letting that affect any currently running audio streams - of course we let each know...) . Had no idea ROON uses multicast (nothing inherently wrong with that) but determining UDP ports, etc should be interesting. To give you an idea, multicast discovery can be easily degraded on many consumer networking devices, especially when we have Wireless Access Points, Printers, Scanners, Cameras, Smartphones, Roon endpoints all shouting HERE I AM, on even the most modest network....Roon seems to dig deeper into the network stack than most other products we use at home, and that triggers bugs in routers, switches, and endpoints that are not noticeable with less demanding network applications. One notorious example are the interactions between some versions of the Android network stack and how network gear handles multicast and IGMP handling. To make things worse, it looks like consumer networking gear has become increasingly flaky with respect less used networking manoevers such as multicast discovery as more specialized, narrower alternatives such as Zeroconf/mDNS have been adopted by key players like Apple and Google. I've bugged Roon folks gently about moving to a more widely used device/service discovery method than their custom multicast one, it would save them a lot of support trouble and upset users in the long run even though it would be a major development project as they'd have to rip out all their old discovery machinery.
It's not a total coincidence that I moved to semi-pro Ubiquiti/UniFi network gear as I started to depend on Roon. But that has its own hardware and knowledge costs.
I'll do a bit of digging, but here is a super quick tip: If your main router supports a "guest network" - use it! Either connect your audio to it, with all other devices to main, or vice versa - this will help segregate UDP traffic.
EDIT: quick check; Roon *device discovery* uses UDP port 9300. RAAT (Roon Advanced Audio Transport) uses TCP 9100-9200 (each core --> endpoint would use a unique TCP port)