31.2.3 Determining Querier

MLD routers act as either Queriers or Non-Queriers. If multiple routers exist on the same network, one of the routers becomes the Querier, which regularly sends a Multicast Listener Query message.

Each MLD router on the network compares the IPv6 link-local address of the local interface with the IPv6 link-local addresses of the senders of Multicast Listener Query messages received from other MLD routers. If the address of the local interface is lower than any other addresses, the MLD router runs as the Querier. If the address of the local interface is higher than any of other addresses, the MLD router becomes a Non-Querier, and does not send a Multicast Listener Query message.

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 31-4: Determining Querier and Non-Querier

When a router becomes the Querier, it runs as the Querier until a Multicast Listener Query with a source IPv6 address lower than the local interface is received, regularly sending Multicast Listener Queries (every 125 seconds by default). A router running as a Non-Querier can become the Querier in the following cases:

• When a Non-Querier monitoring Multicast Listener Query messages sent from the Querier receives a Multicast Listener Query message with the source IPv6 link-local address higher than the local address of the local interface

• When no Multicast Listener Query messages are received within a set time (255 seconds by default)

Determination of the Querier uses only IPv6 link-local addresses that are set on interfaces. Other addresses have no effect on determining the Querier.

The Querier determination method described above is common to both MLDv1 and MLDv2.