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.

Related Tutorials
Unicast, Multicast and Broadcast
Broadcast domain
Broadcast MAC Address - ff:ff:ff:ff:ff:ff
IPv4 multicast MAC Addresses
IPv4 Protocol, IPv4 header and fields of IPv4 header
IPv4 addresses
Class D multicast IP addresses
IPv4 link-local multicast addresses
IPv4 Internetwork control block multicast addresses
IPv4 Source-Specific Multicast (SSM) address reservation
IPv4 GLOP multicast addresses
Administratively scoped multicast address block or Limited scope addresses
Multicast IPv4 address to MAC address mapping
TCP/IP Encapsulation and Decapsulation
What is multicast
What is multicast group
Advantages and disadvantages of multicast
Comparison of multicast with unicast and broadcast
Advantages and disadvantages of multicast
How IPv4 multicast works on LAN
IGMP, What is IGMP, How IGMP works
IGMP message types
IGMPv1 - Internet Group Management Protocol Version 1
IGMPv2 - Internet Group Management Protocol Version 2
What are SSM (Source-specific Multicast) and ASM (Any-source Multicast)
IGMPv3 - Internet Group Management Protocol Version 3