Lots of people seem to be confused by multicast, in general, and specifically how the WLC comes into the mix. This document is meant to be an overview of what needs to be done for the WLC to participate in multicast. It is by no means an indept configuration for a networks multicast configuration.
So to that end, here we go.
First, let's start with the two modes the controllers operate with:
Multicast - Unicast
This is the easiest method to use. When the WLC receives a Multicast packet, it replays that packet to every AP that is connected. Now this does work, but can be very network intense, as every AP gets the stream. So if you have 100 AP, there are 100 streams, 300 AP 300 streams.
Multicast - Multicast
This is the better method to use. With this method, the AP will join to a multicast group, that you configure, preferably in the 239.x.x.x administratively scoped space. Now when the WLC gets a multicast packet, it replays it once to the group.
Select Multicast - Multicast and configure your group, each WLC in your mobility group should use a unique address. For the WLC you are done.
you need to enable multicast routing, if it's not enabled already, and you need to pim enable, the following VLANs. Management, AP-Manger, all the VLAN that the AP are in, as well as the VLAN(s) where the clients that will receive the stream will be.
In my lab, VLAN 20 is the management, VLAN 10 is for AP, and VLAN 30 is where my clients are. So under all of these SVI I need to issue the multicast commands.
You should be able to issue show ip mroute, show ip igmp groups, and all the other multicast commands to validate that the group for the AP is built properly.
On the WLC, there is also the ability to enable IGMP snooping. With this enabled, the WLC will hold it's own snooping table for the IGMP messages that it receives, so that it knows who needs to get the stream.