Normally, a switch should forward multicast frames out all ports. It will forward the traffic to all receivers, but it will also forward it to many hosts that don’t need it. To optimize this behavior, Cisco routers implement IGMP Snooping. With IGMP Snooping, a switch listens to IGMP JOINs and LEAVEs to know which ports should receive traffic for a certain multicast destination.
IGMP Snooping is on by default for all VLANs, If disabled, you can re-enable it with: