15.2.3 Determining Querier

IGMP routers act as either Queriers or Non-Queriers. If multiple routers exist on the same network, a Querier for regularly sending Membership Query messages is determined by comparing the sender IP address and local interface IP address of each Membership Query received from IGMP routers on the same network, and the one with the lower local interface address runs as the Querier. The one with the higher local interface address becomes a Non-Querier, and does not send Membership Queries. This means that only one Querier can exist on the same network. The following figure shows how the Querier and Non-Querier routers are determined.

Figure 15-5: Determining Querier and Non-Querier

When a router becomes the Querier, it runs as the Querier until a Membership Query with a sender IP address lower than the local interface is received, regularly sending Membership Queries (every 125 seconds). A Non-Querier performs monitoring by receiving Querier Membership Queries, and when a Membership Query with sender IP address higher than the local interface is received, or no Membership Queries are received within a set time (255 seconds), it becomes the Querier.

For interfaces configured for multihoming, Querier and Non-Querier are determined for every network configured for the interface.