Reference Guide

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
The following describes how a host can join a multicast group.
1. 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.
2. 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 Adjusting Query and Response
Timers).
3. 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.
Leaving a Multicast Group
The following describes how a host can leave a multicast group.
1. 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.
2. 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.
3. Any remaining hosts respond to the query according to the delay timer mechanism (refer to Adjusting Query and
Response Timers). 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 group from the
list associated with forwarding port and stops forwarding traffic for that group to the subnet.
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