3Com Switch 8800 Advanced Software V5 Configuration Guide

638 CHAPTER 44: MLD CONFIGURATION
■ General query: an IPv6 multicast router or routing switch sends periodical
general queries to determine what IPv6 multicast addresses have active
listeners on the local subnet.
■ Multicast-address-specific query: an IPv6 multicast router or routing switch
sends multicast-address-specific queries to determine whether any listeners for
particular IPv6 multicast addresses exist on the local subnet.
MLD querier election
Of multiple IPv6 multicast routers on the same subnet, all the routers can hear
MLD listener report messages (often referred to as reports) from hosts, but only
one router is needed for sending MLD query messages (often referred to as
queries). So, a querier election mechanism is required to determine 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 referred to as general queries) to all hosts and routers on
the local subnet (the destination address is FF02::1).
2 Upon hearing 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, known as "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 the querier to have timed out and initiates a new
querier election process.
Joining an IPv6 multicast group
Figure 193 How hosts use MLD to join IPv6 multicast groups
Assume that Host B and Host C are expected to receive IPv6 multicast data
addressed to IPv6 multicast group G1, while Host A is expected to receive IPv6
Query
Report
Querier
Host A
(G2)
Host B
(G1)
Host C
(G1)
Ethernet
Router A Router B