MLD Querier is similar to IGMP querier. A multicast query router communicates with hosts on a local network by sending MLD queries. This router periodically sends a general query message to each local network of the router. This is standard multicast behavior.
Note
Queries are sent only if PIM is enabled globally and on the interface. PIM and snooping cannot be enabled at the same time.
Each VLAN using MLD multicast must have a router performing multicast queries. Networks with no stand-alone devices currently have no capability for implementing the pruning of multicast traffic. A dedicated querier must be available on the network.
There are several behavioral differences between a traditional query router and a switch or stack using the MLD Querier functionality. The following are the differences:
There is no election process. When a switch or stack restarts, queries are sent as part of MLD startup. This process stops other devices from sending queries while they detect the new device starting up. The last active device sending queries on the network is the active one. This is not the case with Layer 3 MLD behavior.
If the current active device stops sending queries, a timeout period must elapse before another device takes over. This can result in an ageout of groups, and subsequent flooding, before a new query is sent and the pruning process restarts. This occurs only during the transition between active query devices. Once the new device is established, queries are sent as configured in the Query Interval and Robust Values fields.
Multiple active query devices are not supported. Enabling multiple devices establishes one active device and other devices listening to take over should the active device fail.
The querier version is determined by the received query version and establishes the interface operational version. By default, the interface operational version is MLDv1. If the interface operational version is downgraded from MLDv2 to MLDv1 (when operational version is MLDv2 and a MLDv1 query is received), then all MLDv2 listeners (registered by MLDv2 reports) are removed and all incoming MLDv2 reports are dropped.