Allied Telesis AT-S63 - Manual (Page 200)

Chapter 17: Internet Group Management Protocol Snooping
200 Section III: Snooping Protocols
Without IGMP snooping a switch would have to flood multicast packets out
all of its ports, except the port on which it received the packet. Such
flooding of packets can negatively impact network performance.
The AT-9400 Switch maintains its list of multicast groups through an
adjustable timeout value, which controls how frequently it expects to see
reports from end nodes that want to remain members of multicast groups,
and by processing leave requests.
Note
The default setting for IGMP snooping on the switch is disabled.