3Com 4800G Series, Configuration Manual

Page: 614 / 1246

614 C
HAPTER
46: IGMP C
ONFIGURATION
Figure 184 Joining multicast groups
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 184. The basic process that the hosts join the
multicast groups is as follows:
1 The IGMP querier (Router B in the figure) periodically multicasts IGMP queries
(with the destination address of 224.0.0.1) to all hosts and routers on the local
subnet.
2 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.
3 Host C, which is on the same subnet, hears the report from Host B for joining G1.
Upon hearing the report, Host C will suppress itself from sending a report message
for the same multicast group, 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 over 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 generate (*, 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 with the destination
Query
Report
DR
Host A
(G2)
Host B
(G1)
Host C
(G1)
Ethernet
Router A Router B