102
In IGMPv1, the designated router (DR) elected by the working multicast routing protocol (such as PIM)
serves as the IGMP querier.
For more information about DR, see "Configuring PIM (available only on the S5500-EI)."
Figure 33 IGMP queries and reports
Assume that Host B and Host C are interested in multicast data addressed to multicast group G1, and
Host A is interested in multicast data addressed to G2, as shown in Figure 33. T
he following process
describes how the hosts join the multicast groups and how the IGMP querier (Router B in the figure)
maintains the multicast group memberships:
1. The hosts send unsolicited IGMP reports to the addresses of the multicast groups that 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 delay timer of whichever expires first) sends
an IGMP report to the multicast group address of G1, to announce its membership for G1. Assume
that Host B sends the report message. After receiving the report from Host B, Host C (which is on
the same subnet as Host B) suppresses its own report for G1, because the IGMP routers (Router A
and Router B) have already known that at least one host on the local subnet is interested in G1. This
IGMP report suppression mechanism helps reduce traffic on the local subnet.
4. At the same time, because Host A is interested in G2, it sends a report to the multicast group
address of G2.
5. Through the query/report process, the IGMP routers determine that members of G1 and G2 are
attached to the local subnet, and the multicast routing protocol that is running on the routers (PIM,
for example) generates (*, G1) and (*, G2) multicast forwarding entries. These entries will be the
basis for subsequent multicast forwarding, where asterisk represents any multicast source.
Query
Report
DR
Host A
(G2)
Host B
(G1)
Host C
(G1)
Ethernet
Router A Router B
IP network
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