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Joining an IPv6 multicast group
Figure 107 MLD queries and reports
As shown in
, Host B and Host C want to receive IPv6 multicast data addressed to IPv6
multicast group G1, and Host A wants to receive the 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
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.
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 G1 has at least one member on
the local subnet, other members do not need to report their memberships. 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. The IPv6 multicast routing protocol (for example, IPv6 PIM) 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, the router looks
up the IPv6 multicast forwarding table. Based on the (*, G1) and (*, G2) entries, the router
forwards the IPv6 multicast data to the local subnet. Then, the receivers on the subnet receive
the data.
Leaving an IPv6 multicast group
When a host leaves a multicast group, the following occur:
1.
The host sends an MLD done message to all IPv6 multicast routers on the local subnet. The
destination address is FF02::2.
Query
Report
Querier
Host A
(G2)
Host B
(G1)
Host C
(G1)
Ethernet
Router A
Router B
IPv6 network
