3Com Switch 8800 Advanced Software V5 Configuration Guide

MLD Overview 639
multicast data addressed to G2, as shown in Figure 193. The basic process that the
hosts join the IPv6 multicast groups is as follows:
1 The MLD querier (Router B in the figure) periodically multicasts MLD queries (with
the destination address of FF02::1) to all hosts and routers on the local subnet.
2 Upon receiving a query message, Host B or Host C (the delay timer of whichever
expires first) sends an MLD report to the IPv6 multicast group address of G1, to
announce its interest in G1. Assume it is Host B that sends the report message.
3 Host C, which is on the same subnet, hears the report from Host B for joining G1.
Upon hearing the report, Host C will suppress itself from sending a report message
for the same IPv6 multicast group, because the MLD routers (Router A and Router
B) already know that at least one host on the local subnet is interested in G1.
4 At the same time, because Host A is interested in G2, it sends a report to the IPv6
multicast group address of G2.
5 Through the above-mentioned query/response process, the MLD routers learn that
members of G1 and G2 are attached to the local subnet, and generate (*, G1) and
(*, G2) multicast forwarding entries through an IPv6 multicast routing protocol
(such as IPv6 PIM), which will be the basis for subsequent IPv6 multicast
forwarding, where * represents any multicast source.
6 When the IPv6 multicast data addressed to G1 or G2 reaches an MLD router,
because the (*, G1) and (*, G2) multicast forwarding entries exist on the MLD
router, the router forwards the IPv6 multicast data to the local subnet, and then
the receivers on the subnet receive the data.
Leaving an IPv6 multicast group
When a host leaves a multicast group:
1 This host sends a done message to all IPv6 multicast routers (the destination
address is FF02::2) on the local subnet.
2 Upon receiving the leave message, the querier sends a configurable number of
multicast-address-specific queries to the group being left. The destination address
field and group address field of message are both filled with the address of the
IPv6 multicast group being queried.
3 One of the remaining members, if any on the subnet, of the group being queried
should send a report within the time of the maximum response delay set in the
query messages.
4 If the querier receives a report for the group within the maximum response delay
time, it will maintain the memberships of the IPv6 multicast group; otherwise, the
querier will assume that no hosts on the subnet are still interested in IPv6 multicast
traffic addressed to that group and will stop maintaining the memberships of the
group.
MLD Message Types The MLD querier learns the multicast listening states of neighbor interfaces by
sending MLD query messages.
Figure 194 shows the format of an MLDv1 query
message.