Igmp - TP-Link T3700G-28TQ User Manual

11.2 IGMP

Brief Introduction of IGMP
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IGMP stands for Internet Group Management Protocol. It is responsible for the management of IP
multicast members in IPv4, and is used to establish and maintain the multicast member
relationships between the IP host and its directly neighboring multicast routers.
So far, there are three IGMP versions:
IGMPv1(defined in RFC 1112)

IGMPv2(defined in RFC 2236)

IGMPv3(defined in RFC 3376)
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All IGMP versions support ASM model, and IGMPv3 can be directly applied in SSM model.
IGMPv1 Work Mechanism
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IGMPv1 is mainly based on the query-and-response mechanism to manage the multicast group
members.
When there are multiple multicast routers in the subnet, all of them can receive IGMP membership
report message. A specific router needs to be chosen from these routers through the querier
election mechanism, and it will works as the querier to send IGMP query message.
In IGMPv1, the DR (Designated Router) is elected according to the multicast routing protocol
(such as PIM) as the exclusive IGMP querier to forward the multicast information.
As shown in Figure 11-3, Suppose Host B and Host C expect to receive the multicast traffic
sending to multicast group G1, and Host A expects to receive the multicast traffic sending to
multicast group G2. The basic process of the host joining the multicast group and the IGMP
querier (Router B) maintaining the multicast group membership is as below:
(1) Instead of waiting for the IGMP query message from the IGMP querier, the host will actively
send IGMP membership report message to the multicast group it wants to join in.
(2) The IGMP querier will periodically send the IGMP query message to all the hosts and routers
in the local network with the multicast address 224.0.0.1.
Figure 11-3 IGMP Query-and-Response
224