The querier is a multicast router or a switch that periodically asks MLD hosts on the network to verify their multicast join requests. There is one querier for each VLAN, and all switches on the VLAN listen to the responses of MLD hosts to multicast queries and forward or block multicast traffic accordingly.

All of the switches have the querier function enabled by default. If there is another device on the VLAN that is already acting as querier, the switch defers to that querier. If there is no device acting as querier, the switch enters an election state and negotiates with other devices on the network (if any) to determine which one will act as the querier.

The querier periodically sends general queries to MLD hosts on each multicast address that is active on the VLAN. The time that the querier waits between sending general queries is known as the query interval; the MLD standard sets the default query interval to 125 seconds.

Network nodes that wish to remain active as MLD hosts respond to the queries with join requests; in this way they continue to assert their presence as MLD hosts. The switch through which any given MLD host connects to the VLAN sees the join requests and continues forwarding traffic for that multicast address to the MLD host's port.