How Mldv2 Works - HP 4800G Series Configuration Manual

in Figure 1-1. The following describes how the hosts join the IPv6 multicast groups and 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) 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 membership for
G1. Assume it is Host B that sends the report message. Upon hearing the report from Host B, Host
C, which is on the same subnet with Host B, suppresses its own report for G1, because the MLD
routers (Router A and Router B) already know that at least one host on the local subnet is
interested in G1. This mechanism, known as 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 above-mentioned 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) running on the routers generates (*, G1) and (*, G2) multicast forwarding entries, which
will be the basis for subsequent IPv6 multicast forwarding, where * 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:
1) This host sends an MLD done message to all IPv6 multicast routers (the destination address is
FF02::2) on the local subnet.
2) Upon receiving the MLD done 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 the 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.

How MLDv2 Works

Compared with MLDv1, MLDv2 provides the following new features:
IPv6 multicast group filtering
MLDv2 has introduced IPv6 multicast source filtering modes (Include and Exclude), so that a host can
specify a list of IPv6 multicast sources it expect or does not expect IPv6 multicast data from when it joins
an IPv6 multicast group:
1-3