H3C Technologies H3C S12500 Series Switches - Manual (Page 358)

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Joining an IPv6 multicast group
Figure 91 MLD queries and reports
As shown in Figure 91 , assume that Host B and Host C receive IPv6 multicast data addressed to IPv6
multicast group G1, and Host A receive IPv6 multicast data addressed to G2. The 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 want
to 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.
3. After 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 membership for G1.
Assume that Host B sends the report message. After hearing the report from Host B, Host C, which
is on the same subnet as Host B, suppresses its own report for G1, because the MLD routers (Router
A and Router B) have already known that at least one host on the local subnet is interested in G1.
This mechanism, known as the "MLD report suppression", helps reduce traffic on the local subnet.
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 query/report process, the MLD routers learn that members of G1 and G2 are
attached to the local subnet, and the IPv6 multicast routing protocol (IPv6 PIM for example) that is
running on the routers generates (*, G1) and (*, G2) multicast forwarding entries. These entries
will be the basis for subsequent IPv6 multicast forwarding, where the asterisk (*) represents any
IPv6 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, the following process occurs:
Query
Report
Querier
Host A
(G2)
Host B
(G1)
Host C
(G1)
Ethernet
Router A Router B
IPv6 network