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6. Interoperation with MLDv1

MLDv2 hosts and routers interoperate with hosts and routers that have not yet been upgraded to MLDv2. This compatibility is maintained by the periodic Multicast Listener Query messages that are sent by the Querier, and by the MLDv1 Multicast Listener Report messages that are sent by older hosts.

6.1. MLDv2 Host Operation​

The behavior of an MLDv2 host depends on whether the Querier on the network is speaking MLDv2 or MLDv1.

  • MLDv1 Querier Present: If an MLDv2 host receives an MLDv1 Query, it must respond with MLDv1 Reports. Source filtering is not supported in this mode.
  • MLDv2 Querier Present: If an MLDv2 host receives an MLDv2 Query, it responds with MLDv2 Reports. Source filtering is supported.

The host maintains a compatibility mode variable for each interface. If an MLDv1 Query is received, the host switches to MLDv1 compatibility mode and sets a timer. When the timer expires, the host switches back to MLDv2 mode.

6.2. MLDv2 Router Operation​

The behavior of an MLDv2 router depends on whether there are older hosts or routers on the network.

6.2.1. Presence of MLDv1 Hosts​

An MLDv2 router may receive MLDv1 Reports from older hosts.

  • MLDv1 Report Received: The router acts as if it received an MLDv2 IS_EX({}) report for the group. This means the group is in EXCLUDE mode with an empty source list (i.e., interested in all sources).

When MLDv1 hosts are present, the router must suppress MLDv2-specific processing (like source-specific queries) for the affected groups to ensure compatibility.

6.2.2. Presence of MLDv1 Routers​

If an MLDv2 router is on a network with MLDv1 routers, the Querier election process determines which router becomes the Querier.

  • If an MLDv1 router becomes the Querier, all routers (including MLDv2 routers) must act as MLDv1 routers.
  • If an MLDv2 router becomes the Querier, it sends MLDv2 Queries. However, it must be configured to support MLDv1 hosts if they are present.
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