H3C Technologies H3C S7500E Series Switches - Manual (Page 110)

5-3
Figure 5-1 . The 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.
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 above-mentioned query/report process, the IGMP routers learn that members
of G1 and G2 are attached to the local subnet, and the multicast routing protocol (PIM for
example) running on the routers generates (*, G1) and (*, G2) multicast forwarding entries,
which will be the basis for subsequent multicast forwarding, where * represents any
multicast source.
6) When the multicast data addressed to G1 or G2 reaches an IGMP router, because the (*,
G1) and (*, G2) multicast forwarding entries exist on the IGMP router, the router forwards
the multicast data to the local subnet, and then the receivers on the subnet receive the
data.
As IGMPv1 does not specifically define a Leave Group message, upon leaving a multicast
group, an IGMPv1 host stops sending reports to the address of the multicast group it listened to.
If no member of a multicast group exists on the subnet, the IGMP router will not receive any
report addressed to that multicast group, so the routers will delete the multicast forwarding
entries for that multicast group after a period of time.
Enhancements in IGMPv2
Compared with IGMPv1, IGMPv2 has introduced a querier election mechanism and a
leave-group mechanism.
Querier election mechanism
In IGMPv1, the DR elected by the Layer 3 multicast routing protocol (such as PIM) serves as
the querier among multiple routers on the same subnet.
In IGMPv2, an independent querier election mechanism is introduced. The querier election
process is as follows:
1) Initially, every IGMPv2 router assumes itself as the querier and sends IGMP general query
messages (often referred to as general queries) to all hosts and routers on the local subnet
(the destination address is 224.0.0.1).