The IGMP-SSM Mapping feature allows IGMPv1 and IGMPv2 hosts to participate in SSM functionality, and eliminates the need for IGMPv3. You can configure SSM map entries that specify the sources used for a group/group range for which SSM functionality has to be applied. You also have the option to configure the domain name and DNS server to use to obtain the source list.
When a IGMPv1 or IGMPv2 report is received, the configured sources are provided to PIM so that it can send source specific joins. When the host leaves or when the membership times out, PIM is informed so that it can consider sending prunes.
In a multi-access network (where more than one router is receiving IGMP messages from the hosts), only the designated router sends joins towards the source, so it is desirable to have same configuration for SSM group range and SSM Mapping range on all routers in a VLAN.