I have read many posts regarding this and see that there are many answers to many questions. So please bear with me if I have missed an obvious solution somewhere.
We have a training centre with multiple rooms, each room has been allocated a port one 3COM 5500-E gig (backbone) switch and that port has been allocated into a VLAN also setup on the 3COM switch. We have a DHCP server distributing an IP scope for each room. This is also in its own VLAN on a trunk port on the 3com switch.
Everything DHCP wise seems to be working as expected, when you attempt a ghost multicast session, the client connects to the ghost server and then hangs. Unicasts work.
If you move the port allocated to the room on the 3com switch into the Ghosting VLAN, as you would expect, it works. However, all the rooms must have their own VLAN otherwise the ghosting floods the network.
IGMP snooping is enabled on the room switches (3com 2400) as well as the 3com 5500-e switch.