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Configuring IGMP
Overview
Internet Group Management Protocol (IGMP) establishes and maintains the multicast group
memberships between a Layer 3 multicast device and the hosts on the directly connected subnet.
IGMP has the following versions:
IGMPv1 (defined by RFC 1112).
IGMPv2 (defined by RFC 2236).
IGMPv3 (defined by RFC 3376).
All IGMP versions support the ASM model. IGMPv3 can directly implement the SSM model. IGMPv1
and IGMPv2 must work with the IGMP SSM mapping feature to implement the SSM model. For more
information about the ASM and SSM models, see "Multicast overview."
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 23 IGMP queries and reports
As shown in Figure 23, 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 23)
maintains the multicast group memberships:
Query
Report
DR
Host A
(G2)
Host B
(G1)
Host C
(G1)
Ethernet
Router A Router B
IP network
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