Overview of MLD Snooping
Definition
Multicast Listener Discovery Snooping (MLD snooping) is an IPv6 Layer 2 multicast protocol. The MLD snooping protocol maintains information about the outbound interfaces of multicast packets by snooping multicast protocol packets exchanged between the Layer 3 multicast device and user hosts. MLD snooping manages and controls multicast packet forwarding at the data link layer.
Purpose
Similar to an IPv4 multicast network, multicast data on an IPv6 multicast network (especially on an LAN) have to pass through Layer 2 switching devices. As shown in Figure 10-1, a Layer 2 switch locates between multicast users and the Layer 3 multicast device, Router.
After receiving multicast packets from Router, Switch forwards the multicast packets to the multicast receivers. The destination address of the multicast packets is a multicast group address. Switch cannot learn multicast MAC address entries, so it broadcasts the multicast packets in the broadcast domain. All hosts in the broadcast domain will receive the multicast packets, regardless of whether they are members of the multicast group. This wastes network bandwidth and threatens network security.
MLD snooping solves this problem. MLD snooping is a Layer 2 multicast protocol on the IPv6 network. After MLD snooping is configured, Switch can snoop and analyze MLD messages between multicast users and Router. The Layer 2 multicast device sets up Layer 2 multicast forwarding entries to control forwarding of multicast data. In this way, multicast data is not broadcast on the Layer 2 network.