HP 4800G Series Configuration Manual page 789

Of multiple multicast routers on the same subnet, all the routers can hear IGMP membership report
messages (often referred to as reports) from hosts, but only one router is needed for sending IGMP
query messages (often referred to as queries). So, a querier election mechanism is required to
determine which router will act as the IGMP querier on the subnet.
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, refer to PIM Configuration in the IP Multicast Volume.
Figure 1-1 IGMP queries and reports
Router A
Ethernet
Host A
(G2)
Query
Report
Assume that Host B and Host C are expected to receive multicast data addressed to multicast group G1,
while Host A is expected to receive multicast data addressed to G2, as shown in
following describes how the hosts join the multicast groups and 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) Upon 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 it is Host B that sends the report message. Upon hearing the report from Host B, Host C,
which is on the same subnet with Host B, suppresses its own report for G1, because the IGMP
routers (Router A and Router B) already know that at least one host on the local subnet is
interested in G1. This mechanism, known as IGMP report suppression, helps reduce traffic on the
local subnet.
IP network
DR
Router B
Host B
(G1)
Host C
(G1)
1-2
Figure 1-1. The