Hello Shinichi,

your understanding is correct:

router takes care of Last reporter only for each group.

From the router point of view it is interested to know if there is at least one host interested in the group on the interface to put in Oilist.

By the way, even if the router could try to record all receivers its info would be incomplete for the IGMP report suppression feature used by IGMP v2.

A host suppresses its own report if heards another IGMP report message from another host for the same group G.

The only devices that can have an accurate picture are lan switches using IGMP snooping or older CGMP.

But you need to go to OSI L2 to find out what ports are currently associated to a multicast MAC address.

See this example from a C7609

sh mac-address-table multicast vlan 617 | beg 0100.5e48.6403

617 0100.5e48.6403 static Yes - Po1,Po277,Router

617 0100.5e48.6402 static Yes - Po1,Po277,Router

617 0100.5e48.6406 static Yes - Po1,Po277,Router

617 0100.5e00.0128 static Yes - Po1,Po277,Router

Here vlan617 is in a VRF, po1 is bundle to other C7609, po277 is internal bundle to FWSM (using a transparent context)

Note: things are different with IGMPv3

IGMPv3 has a feature called explicit-tracking and doesn't support report suppression (for the hosts joining (S,G) telling also the Source )

sh ip igmp snooping ?

>> explicit-tracking IGMPv3 Explicit tracking database

mrouter Show routers beyond a port

rate-limit IGMP messages incoming rate

statistics IGMPv3 channel statistics

So with IGMPv3 a router potentially could build a complete picture using this feature.

Hope to help

Giuseppe