IGMP/MLD snooping runs on Layer 2 devices. It listens to IGMP or MLD packets between Layer 3 devices and hosts to generate a Layer 2 multicast forwarding table (MDB, Multicast Database Table), thereby managing and controlling multicast data packet forwarding and implementing on-demand distribution of multicast data packets at Layer 2.
ACOS supports enabling IGMP/MLD snooping at the virtual distributed switch (VDS) level. After IGMP/MLD snooping is enabled, the OVS bridge corresponding to the VDS listens to IGMP/MLD packets sent by connected virtual machines or external devices, and generates an MDB table to control multicast packet forwarding.
Packets whose destination multicast addresses are in the MDB table are forwarded only to virtual machines that have joined the corresponding multicast groups.
Packets whose destination multicast addresses are not in the MDB table are dropped and not forwarded to any virtual machine on the node.
If the application traffic in the virtual distributed switch is multicast traffic but does not use IGMP/MLD packets for group membership management, do not enable this feature. Otherwise, such multicast packets cannot be forwarded.
After IGMP/MLD snooping is enabled, to prevent MDB entries from expiring, the mcast-agent component on the ACOS node periodically sends IGMP/MLD Query packets to virtual machines on the node. The virtual machines respond with Report packets to refresh the corresponding entries.
