how is it possible to connect different vlans to a multicast address?
With a multicast router.
And what protocol, your next question will be, does one route multicast with? Answer: PIM-SM
RouterOS also has a proprietary IGMP proxy service
that may be of use here.
There have been reports of bugs with these facilities in ROS 7, but since I don't use those, I can't corroborate that. For all I know, they're all fixed now, and if not, then maybe the remaining bugs don't affect your use case. Give them a try, and if it doesn't work, contact support
a daemon running that listens on 184.108.40.206.
Presumably a KNX automation controller
If not, someone's abusing the IANA assigned IP space.
I temporarily fixed it by putting both systems in the same VLAN/subnet. But that doesn't sound right.
Why not? You have a controller and an application that speaks to the controller. Why must there be a VLAN barrier between them?
I'd understand if you were talking about random Chinese IoT home automation crap, but this KNX stuff looks high-end. Surely you can assume there aren't current vulnerabilities in this equipment, and that it gets regular patches. If not, why pay the prices they must be charging to produce that huge web site as a mere marketing ancillary?
Define your threat model, and the proper response will fall out of that.