IGMPv1 overview
IGMPv1 manages multicast group memberships based on the query and response mechanism.
All routers that run IGMP on the same subnet can get IGMP membership report messages (called reports) from hosts. However, only one router can act as the IGMP querier to send IGMP query messages (called queries). The querier election mechanism determines which router acts as the IGMP querier on the subnet.
In IGMPv1, the DR elected by the multicast routing protocol (such as PIM) acts as the IGMP querier. For more information about DR, see "Configuring PIM."
Figure 31: IGMP queries and reports
As shown in Figure 31, Host B and Host C are interested in the multicast data addressed to the multicast group G1. Host A is interested in the multicast data addressed to G2. The following process describes how the hosts join the multicast groups and how the IGMP querier (Router B in Figure 31) maintains the multicast group memberships:
The hosts send unsolicited IGMP reports to the multicast groups they want to join without having to wait for the IGMP queries.
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.
After receiving a query message, the host whose report delay timer expires first sends an IGMP report to multicast group G1 to announce its membership for G1. In this example, Host B sends the report message. After receiving the report from Host B, Host C suppresses its own report for G1.
Because IGMP routers already know that G1 has a minimum of one member, other members do not need to report their memberships. This mechanism, known as the host IGMP report suppression, helps reduce traffic on the local subnet.
At the same time, Host A sends a report to the multicast group G2 after receiving a query.
Through the query and response process, the IGMP routers (Router A and Router B) determine that the local subnet has members of G1 and G2. The multicast routing protocol (PIM, for example) on the routers generates (*, G1) and (*, G2) multicast forwarding entries, where asterisk (*) represents any multicast source. These entries are the basis for subsequent multicast forwarding.
When the multicast data addressed to G1 or G2 reaches an IGMP router, the router looks up the multicast forwarding table. Based on the (*, G1) or (*, G2) entries, the router forwards the multicast data to the local subnet. Then, the receivers on the subnet can receive the data.
IGMPv1 does not define a leave group message (often called a leave message). When an IGMPv1 host is leaving a multicast group, it stops sending reports to that multicast group. If the subnet has no members for a multicast group, the IGMP routers will not receive any report addressed to that multicast group. In this case, the routers clear the information for that multicast group after a period of time.