Overview of MLD Snooping
Definition
Multicast Listener Discovery (MLD) snooping is a Layer 2 multicast protocol running on IPv6 networks, which listens on multicast protocol packets between a Layer 3 multicast device and user hosts to maintain outbound interfaces of multicast packets. MLD snooping manages and controls multicast packet forwarding at the data link layer.
Purpose
On an IPv6 multicast network (especially on an LAN), multicast data packets need to pass through Layer 2 switches between Layer 3 multicast devices and user hosts, as shown in Figure 1.
Destination addresses of multicast packets are multicast group addresses and cannot be learned by a Layer 2 device (the switch in Figure 1). Therefore, when the switch receives multicast data packets from the router, it broadcasts the packets in the broadcast domain. All hosts in the broadcast domain receive the multicast packets, regardless of whether they are group members. This wastes network bandwidth and brings security risks.
MLD snooping solves this problem on the IPv6 network. With MLD snooping configured, switches can analyze MLD messages exchanged between user hosts and the router to create and maintain a Layer 2 multicast forwarding table. In this way, multicast data packets are forwarded based on the Layer 2 multicast forwarding table instead of being broadcast.