Well.. it is a live network and change mgmt policies are in place. Being able to tell what vlan was the source would have been helpfull.
We did finaly get permission and enabled bcmc-optimization on all vlans. Overall goodput dropped from 200 Mbps with 95% multicast frames to 30 Mbps and 4% multicast frames.
All is good again I hear you say?
Well, it would be but suddenly the guest network can't communicate anymore. They receive an ip address just find but cannot ping their default gateway or anything further up anymore. Pings to the controller are fine.
Disable the bcmc-optimization for that vlan and it works again.. so now I'm breaking my head trying to figure out what bc or mc has to do with a simple ping.