SSM, or Source Specific Multicast, is the simplest form of multicasting. The receiver specifies the list of sources from which it will accept multicast flows, as well as the group address in an MLD Report message (alternatively, there could be an “exclude” report, listing the sources that the host does not want to listen to, but this requires source discovery by using an RP and does not work with SSM). The multicast router builds shortest-path trees toward the source(s) specified in the report, and the senders may start transmitting. There is no need for RPs with SSM. In fact, groups within the IPv6 SSM range will never have an RP mapped to them. The special SSM group range is FF3x::/96. This allows for 232 multicast groups per sender. You may filter the sources that a listener may join by using the MLD configuration command ipv6 mld access-group , where an access-list specifies the sources and the group ranges.
Remember from IPv4 SSM that SSM identifies not only a group but also the source. So an SSM channel would look something like (2002:1:1:1::6, FF31::66) which is different that (2002:1:1:1::9, FF31::66).
For upstream routers, to enable PIM SSM for IPv6 you would use the following:
R1(config)# ipv6 pim ssm default
To act as a receiver and join a SSM channel you would use this:
R1(config)# interface GigabitEthernet0/1
R1(config-if)# ipv6 mld join-group ff31::66 2002:1:1:1::6
R1(config-if)# exit
SSM for IPv6 does require MLDv2, just as SSM for IPv4 required IGMPv3.