Introduction To Igmpv1 - 3Com S7906E Configuration Manual

For more information about the ASM and SSM models, see Multicast Overview of the IP Multicast
Volume.

Introduction to IGMPv1

IGMPv1 manages multicast group memberships mainly based on the query and response mechanism.
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:
IP network
DR
Router B
Host B
(G1)
Host C
(G1)
1-2
Figure 1-1. The