57
Figure 22 IGMP queries and reports
As shown in Figure 22, Host B and Host C are interested in the multicast data addressed to the multicast
group G1. Host A is interested in the multicast data addressed to G2. The following process describes
how the hosts join the multicast groups and how the IGMP querier (Router B in Figure 22) main
t
ains the
multicast group memberships:
1. The hosts send unsolicited IGMP reports to the multicast groups they want to join without having to
wait for the IGMP queries from the IGMP querier.
2. The IGMP querier periodically multicasts IGMP queries (with the destination address of 224.0.0.1)
to all hosts and routers on the local subnet.
3. After receiving a query message, Host B or Host C (the host whose delay timer expires first) sends
an IGMP report to the multicast group G1 to announce its membership for G1. This example
assumes that Host B sends the report message. After receiving the report from Host B, Host C
suppresses its own report for G1. Because IGMP routers (Router A and Router B) already know that
G1 has at least one member (Host B), other members do not need to report their memberships. This
mechanism, known as "IGMP report suppression," helps reduce traffic on the local subnet.
4. At the same time, Host A sends a report to the multicast group G2 after receiving a query message.
5. Through the query and response process, the IGMP routers (Router A and Router B) determine that
the local subnet has members of G1 and G2. The multicast routing protocol (PIM, for example) on
the routers generates (*, G1) and (*, G2) multicast forwarding entries, where asterisk (*)
represents any multicast source. These entries are the basis for subsequent multicast forwarding.
6. When the multicast data addressed to G1 or G2 reaches an IGMP router, the router looks up the
multicast forwarding table. Based on the (*, G1) or (*, G2) entries, the router forwards the
multicast data to the local subnet. Then, the receivers on the subnet can receive the data.
IGMPv1 does not define a leave group message (often called a "leave message"). When an IGMPv1
host is leaving a multicast group, it stops sending reports to that multicast group. If the subnet has no
members for a multicast group, the IGMP routers will not receive any report addressed to that multicast
group. In this case, the routers clear the information for that multicast group after a period of time.
Query
Report
DR
Host A
(G2)
Host B
(G1)
Host C
(G1)
Ethernet
Router A Router B
IP network
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