High density virtual machine setups can have 100 VMs per host. Each VM has at least a link-local address and a routable address. This is 200 groups per port, 9600 per 48 port switch. This is a rather large amount of state for what it's worth. If you have mld snooping on a switch aggregating multiple racks like this, you start hitting limits on some platforms. There is a similar situation with a WLAN that has large amounts of clients; a single AP, on the other hand, should not see that many groups. Multicast always requires state in the whole network for each group, or flooding. In the case of ndp, flooding may very well be the better option, especially if you view this as a DoS to your Really Important multicast groups - some virtual hosters give /64 per VM, which brings about all kinds of trouble not limited to multicast groups if the client decides to configure too many addresses to his server.