HP A5500 SI Switch Series Configuration Manual page 97

Figure 29 IGMP queries and reports
Router A
Ethernet
Host A
(G2)
Query
Report
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
describes how the hosts join the multicast groups and how the IGMP querier—Router B in the figure—
maintains the multicast group memberships:
The hosts send unsolicited IGMP reports to the addresses of the multicast groups that they will join,
1.
without having to wait for the IGMP queries from the IGMP querier.
The IGMP querier periodically multicasts IGMP queries—with the destination address of
2.
224.0.0.1—to all hosts and routers on the local subnet.
After receiving a query message, Host B or Host C—the delay timer of whichever expires first—
3.
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 determined 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.
At the same time, because Host A is interested in G2, it sends a report to the multicast group
4.
address of G2.
Through the query/report process, the IGMP routers determine that members of G1 and G2 are
5.
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 * represents any multicast source.
When the multicast data addressed to G1 or G2 reaches an IGMP router, because the (*, G1) and
6.
(*, 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.
Because IGMPv1 does not specifically define a leave group message—often called a "leave message",
when an IGMPv1 host leaves a multicast group, it stops sending reports to the address of the multicast
group it listened to. If no member exists in a multicast group on the subnet, the IGMP router will not
IP network
DR
Router B
Host B
(G1)
85
Host C
(G1)
Figure 29. The following process