Source Specific Multicast (SSM)
Recall that networks enabled with multicast protocols from the Internet Standard Multicast (ISM) model must maintain knowledge of both sources and groups (S, G) to enable the receiver to subscribe to (*, G) groups without any knowledge of what senders they are receiving the multicast traffic from. With the SSM model however, the receivers must explicitly subscribe and unsubscribe to the (S, G) multicast channels from which they want to receive traffic. Your receivers must specify both the source unicast IP address that the stream is originating from and the multicast
Benefits of SSM over ISM are
The drawbacks to SSM are
Recall that IGMPv3, IGMP v3lite, and URD enable receivers to send join messages that contain the sources and group (S, G) pairs they want to join. Once first-hop routers join the RP shared tree, they switch over to the SPT by sending PIM (S, G) joins directly to the source, creating a SPT from the source to the receivers. The source multicast stream is then forwarded through the SPT from the source directly to the receivers using the PIM (S, G) state tables and RPF.
The PIM-SSM multicast protocol is derived from PIM-SM and includes only the generation and sending of PIM join (S, G) messages destined to sources. PIM (S, G) and (*, G) join messages are no longer generated and sent to RPs by PIM-SSM last-hop routers. First-hop routers no longer generate PIM-SM register messages. Furthermore, routers enabled with PIM-SSM ignore all Auto-RP, BSR, and MSDP-
You can run PIM-SSM concurrently with PIM-SM in your network. To enable SSM in a network already enabled for PIM-SM, you need only enable the last-hop routers with the SSM ip pim ssm interface configuration command on select interfaces. The last-hop routers process PIM messages for groups within the SSM IP address range with PIM-SSM functionality. PIM routers treat all PIM messages for groups outside the SSM range with PIM-SM functionality.
To enable PIM-SSM alone in your network, use the ip pim ssm command on all router interfaces.