The PIM-SSM model provides the source-specific transmission service for receivers. In PIM-SSM, a dedicated multicast forwarding path is set up directly between a source and a receiver.
PIM-SSM adopts partial PIM-SM technologies. In Protocol Independent Multicast-Source-Specific Multicast (PIM-SSM), there is no need for Rendezvous Point (RP) mapping, rendezvous point tree (RPT) construction, or multicast source registration. In PIM-SSM, the Designated router (DR) is valid only on the shared network segment connected with group members. After learning the requirements of users, the receiver's DR sends a Join message to the multicast source. The Join message is sent upstream hop by hop and (S, G) entries are created correspondingly. As a result, a shortest path tree (SPT) from the router directly connected to the source to the receiver's DR is set up.
To implement PIM-SSM, the following conditions must be met:
The multicast group that users join is in the SSM group address range.
Users specify the source when joining a multicast group.
All routers run PIM-SM.
A network can adopt both the PIM-SM and PIM-SSM models. To implement PIM-SSM, first complete PIM-SM configurations, including RP configurations. Then, adjust the SSM group address range as needed. If the groups that hosts want to join are not in the SSM group address range, PIM-SM is adopted to forward multicast data.