Listeners and joins

The "snooping" part of MLD snooping arises because a switch must track which ports have network nodes that are MLD hosts for any given multicast address. It does this by tracking "joins" on a per-port basis.

A network node establishes itself as an MLD host by issuing a multicast "join" request (also called a multicast "report") for a specific multicast address when it starts an application that listens to multicast traffic. The switch to which the node is connected sees the join request and forwards traffic for that multicast address to the node's port.