Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

There are two ways that a host may join a multicast group: it may respond to a general query from its querier, or it may send an unsolicited report to its querier.

• Responding to an IGMP Query.
  • One router on a subnet is elected as the querier. The querier periodically multicasts (to all-multicast-systems address 224.0.0.1) a general query to all hosts on the subnet.
  • A host that wants to join a multicast group responds with an IGMP membership report that contains the multicast address of the group it wants to join (the packet is addressed to the same group). If multiple hosts want to join the same multicast group, only the report from the first host to respond reaches the querier, and the remaining hosts suppress their responses (for how the delay timer mechanism works, refer to IGMP Snooping ).
  • The querier receives the report for a group and adds the group to the list of multicast groups associated with its outgoing port to the subnet. Multicast traffic for the group is then forwarded to that subnet.
• Sending an Unsolicited IGMP Report.
  • A host does not have to wait for a general query to join a group. It may send an unsolicited IGMP membership report, also called an IGMP Join message, to the querier.