IGMP Querier Election

An IGMP-enabled multicast router assumes one of the following two roles on a network segment:

• Querier

  A querier sends IGMP Query messages and receives IGMP Report and Leave messages from hosts. In this way, the querier discovers which multicast groups have receivers (multicast group members) on the network segments connected to the interfaces that receive the messages.

• Non-querier

  A non-querier receives only Report messages from hosts. This helps the non-querier discover which multicast groups have receivers on the network segments connected to the interfaces that receive the messages. The non-querier identifies the members that leave multicast groups based on the querier's behavior.

Each network segment can have only one querier, which is elected by multicast devices on the network segment. The election rules are as follows:

• After IGMP is enabled on a router (Router A), during the IGMP startup process, Router A considers itself to be the querier for the network segment it resides on, and sends IGMP Query messages to other routers on the network segment. If Router A receives an IGMP Query message from a router (Router B) with a smaller IP address, Router A changes into a non-querier, starts the Other Querier Present Timer, and records Router B as the querier of the network segment.

• If a non-querier (Router A) receives an IGMP Query message from the querier (Router B), Router A updates the Other Querier Present Timer. If Router A receives an IGMP Query message from another router (Router C) with a smaller IP address than the querier, Router A changes its local querier setting to Router C and updates the Other Querier Present Timer.

• If the Other Querier Present Timer expires when Router A is in the non-querier state, Router A changes to the querier state and functions as the querier.

IGMPv1 does not support querier election and therefore relies on the Protocol Independent Multicast (PIM) protocol for selecting a querier. Currently, querier election takes place only among routers that run the same IGMP version and reside on the same network segment. For this reason, to ensure that routers on a network segment operate normally, ensure they are configured with the same IGMP version.