Figure 43. IGMP Messages in IP Packets
Join a Multicast Group
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.
Leave a Multicast Group
The following describes how a host can leave a multicast group.
• A host sends a membership report of type 0x17 (IGMP Leave message) to the all routers multicast
address 224.0.0.2 when it no longer cares about multicast traffic for a particular group.
• The querier sends a Group-Specific Query to determine whether there are any remaining hosts in the
group. There must be at least one receiver in a group on a subnet for a router to forward multicast
traffic for that group to the subnet.
• Any remaining hosts respond to the query according to the delay timer mechanism (refer to IGMP
Snooping). If no hosts respond (because there are none remaining in the group), the querier waits a
specified period and sends another query. If it still receives no response, the querier removes the
Internet Group Management Protocol (IGMP) 389