OcNOS supports MLDv1 and MLDv2. By default, OcNOS enables MLDv2 when PIMv6 is enabled on an interface.

MLDv2 includes the following key changes from MLDv1:

MLD works on the premise of three major packets exchange between MLD enabled routers and hosts, interested in joining a particular group.

Once MLD is enabled or PIMv6 is enabled (which enables MLDv2), on any interface it starts sending Query message, which is called general query to the all-hosts multicast group at ff02::1 periodically to discover whether any hosts want to receive multicast data.

OcNOS elects a router as the MLD querier on a subnet if it has the lowest IP address. As long as a router continues to receive query messages from a router with a lower IP address, it resets a timer that is based on its querier timeout value. If the querier timer of a router expires, it becomes the designated querier. If that router later receives a host query message from a router with a lower IP address, it drops its role as the designated querier and sets its querier timer again.

In the figure below Router-1 eth2 sends query every query-interval. Since Router1-eth2 IPv6 link local address is less than Router-2 eth2, Router-1 eth2 becomes querier on the LAN.

When a host receives a query from the local router it sends a Host Membership Report for all the multicast groups for which it wants to receive multicast traffic. This is called solicited membership report.

When a host joins a new group, the host immediately sends a Membership Report to inform a local router that it wants to receive multicast traffic for the group it has just joined without waiting to receive a Query. This is called unsolicited membership report.

In the figure below Host-1 and Host-2 sends membership reports to Router-1 eth2 for all the multicast groups for which they want to receive multicast traffic. Upon reception of membership report Router-1 maintains an MLD group table containing multicast group-address, interface name on which it receives the report.