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I'm new to how IGMP works. Recently, I was looking at a packet capture:

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I see that 172.18.1.3 sends a membership query to the all hosts address, leading me to assume that 172.18.1.3 is a multicast router. I don't understand why the same host sends a membership report. Hopefully someone can help me better understand why this behaviour occurs. Thanks in advance!

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From RFC3376

The Internet Group Management Protocol (IGMP) is used by IPv4 systems (hosts and routers) to report their IP multicast group memberships to any neighboring multicast routers. Note that an IP multicast router may itself be a member of one or more multicast groups, in which case it performs both the "multicast router part" of the protocol (to collect the membership information needed by its multicast routing protocol) and the "group member part" of the protocol (to inform itself and other, neighboring multicast routers of its memberships).

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