IGMP DR and Querier

In multi-access network segments, there are two roles performed by multicast routers. 1) DR (Designated Router) and 2) Querier (the multicast router who generates and forwards IGMP Membership Queries)

Please refer below image.

As we can see from above image, Router 1, Router2 and Router 3 are connected to the same multi-access Ethernet network segment. In above topology (multi-access networks) there is a chance that two routers (here Router 2 and Router 3) forward PIM join messages from Router 1 to Router 4. This may result in duplicate multicast streams.

In IGMPv1, DR and Querier roles are performed by the same router. IGMPv2/IGMPv3 can elect a Querier role and hence in multi-access networks DR and Querier roles are separate. In IGMPv2, the Querier sends IGMP queries to the multi-access network segment. Since IGMP Membership Query (MQ) messages and IGMP Membership Report (MR) messages are multicast messages, all the routers in the multi-access network segment can listen those messages. The Designated Router (DR) also receives the replies from multicast clients and then Designated Router (DR) processes PIM Joins.

IGMP DR and Querier are selected by an election. The router with highest IPv4 address becomes the DR and the router with lowest IPv4 address becomes the IGMP Querier.