In MLD group compatibility mode, MLDv2 multicast devices are compatible with MLDv1 hosts. An MLDv2 multicast device can process Multicast Listener Report messages of MLDv1 hosts. When an MLDv2 multicast device that supports MLD group compatibility receives Multicast Listener Report messages from an MLDv1 host, the MLDv2 multicast device automatically changes its MLD version to MLDv1 and operates in MLDv1. Then, the MLDv2 multicast device ignores MLDv2 BLOCK messages and the multicast source list in the MLDv2 TO_EX messages. The multicast source-selecting function of MLDv2 messages is therefore suppressed.
If you manually change the MLDv1 version of a multicast device to the MLDv2 version, the multicast device still operates in the MLDv1 version if MLDv1 group members exist. The multicast device upgrades to the MLDv2 version only after all MLDv1 group members leave.