Allied Telesis AT-S63 - Manual (Page 383)

AT-S63 Management Software Menus Interface User’s Guide
Section III: IGMP Snooping, MLD Snooping, and RRP Snooping 383
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 Series 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
By default, IGMP snooping is disabled on the switch.