Queries and reports stay on the local link
IPv4 multicast routers query their local network to learn which groups have listeners. Hosts send membership reports as their interest changes and in response to queries. IGMPv2 adds a Leave Group message, which lets a router check whether any listeners remain before removing membership.
IGMP is local host-to-router signaling. A multicast routing protocol such as PIM uses that membership to build a forwarding tree between routers. A switch may also snoop IGMP to limit which ports receive multicast frames within a VLAN.
IGMPv3 adds source filtering
An IGMPv3 receiver can report INCLUDE mode to request traffic from selected sources, or EXCLUDE mode to receive traffic from all sources except those listed. An INCLUDE report naming one source and one group can request a source-specific multicast channel.
Try it in a lab
- On a receiver LAN with an IGMPv3-capable router and host, start a single receiver for an SSM group and name its source explicitly.
- Capture the receiver’s IGMPv3 report and identify the group and source in its group record. Confirm that the router shows local membership for the same source and group.
- Close the only receiver. Observe the membership change and verify that the router stops forwarding that channel to the LAN after processing it.