MLD operation

Multicast communication can take place without MLD, and by default, MLD is disabled. In that case, if a switch receives a packet with a multicast destination address, it floods the packet to all ports in the same VLAN (except the port that it came in on), as shown in the following figure. Any network nodes that are listening to that multicast address will see the packet; all other hosts ignore the packet.

Figure 2: Without MLD, multicast traffic is flooded to all ports

When MLD snooping is enabled on a VLAN, the switch acts to minimize unnecessary multicast traffic. If the switch receives multicast traffic destined for a given multicast address, it forwards that traffic only to ports on the VLAN that have MLD hosts for that address, as shown in the following figure. It drops that traffic for ports on the VLAN that have no MLD hosts (except for a few special cases explained below).

Figure 3: With MLD snooping, traffic is sent to MLD hosts