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MLD querier election
All IPv6 multicast routers on the same subnet can monitor MLD listener report messages (often called
"reports") from hosts, but only one router is needed to send MLD query messages (often called "queries").
The querier election mechanism determines which router will act as the MLD querier on the subnet.
1. Initially, every MLD router assumes itself as the querier and sends MLD general query messages
(often called "general queries") to all hosts and routers on the local subnet. The destination address
is FF02::1.
2. After receiving a general query, every MLD router compares the source IPv6 address of the query
message with its own interface address. After comparison, the router with the lowest IPv6 address
wins the querier election and all other routers become non-queriers.
3. All the non-queriers start a timer called the "other querier present timer." If a router receives an
MLD query from the querier before the timer expires, it resets this timer. Otherwise, it assumes that
the querier has timed out and initiates a new querier election process.
Joining an IPv6 multicast group
Figure 83 MLD queries and reports
Assume that Host B and Host C will receive IPv6 multicast data addressed to IPv6 multicast group G1,
and Host A will receive IPv6 multicast data addressed to G2, as shown in Figure 83. T
he following
process describes how the hosts join the IPv6 multicast groups and how the MLD querier (Router B in the
figure) maintains the IPv6 multicast group memberships:
1. The hosts send unsolicited MLD reports to the addresses of the IPv6 multicast groups that they will
join, without having to wait for the MLD queries from the MLD querier.
2. The MLD querier periodically multicasts MLD queries (with the destination address of FF02::1) to
all hosts and routers on the local subnet.
Query
Report
Querier
Host A
(G2)
Host B
(G1)
Host C
(G1)
Ethernet
Router A Router B
IPv6 network
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