H3C S7500E Series Ethernet Switches Operation Manual

Operation Manual – Multicast
H3C S7500E Series Ethernet Switches Chapter 4 IGMP Configuration
4-2
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 a multicast routing protocol (such as
PIM) serves as the IGMP querier.
 Note:
For more information about DR, refer to DR election.
Query
Report
DR
Host A
(G2)
Host B
(G1)
Host C
(G1)
Ethernet
Router A Router B
IP network
Figure 4-1 IGMP query and 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
Figure 4-1. The basic process that the hosts join the multicast groups and
the IGMP querier (Router B in the figure) maintains multicast group membership is as
follows:
1) The hosts do not need to wait for a query message from the IGMP querier. Instead,
they send unsolicited IGMP reports to the addresses of the multicast groups that
they want to join.
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 interest in G1. Assume it is Host B that sends the report message.