PIM-SSM overview

The ASM model includes PIM-DM and PIM-SM. The SSM model can be implemented by leveraging part of the PIM-SM technique. It is also called "PIM-SSM."

The SSM model provides a solution for source-specific multicast. It maintains the relationship between hosts and routers through IGMPv3.

In actual applications, part of IGMPv3 and PIM-SM techniques are adopted to implement the SSM model. In the SSM model, because receivers have located a multicast source, no RP or RPT is required and multicast sources do not register for discovering multicast sources in other PIM domains.

Neighbor discovery

PIM-SSM uses the same neighbor discovery mechanism as PIM-SM. For more information, see "Neighbor discovery."

DR election

PIM-SSM uses the same DR election mechanism as PIM-SM. For more information, see "DR election."

SPT building

The decision to build an RPT for PIM-SM or an SPT for PIM-SSM depends on whether the multicast group that the multicast receiver joins is in the SSM group range. The SSM group range reserved by IANA is 232.0.0.0/8.

Figure 31: SPT building in PIM-SSM

As shown in Figure 31, Host B and Host C are receivers. They send IGMPv3 report messages to their DRs to express their interest in the multicast information that the multicast source S sends to the multicast group G.

After receiving a report message, the DR first checks whether the group address in this message is in the SSM group range and does the following:

In PIM-SSM, the term "channel" refers to a multicast group, and the term "subscribe message" refers to a join message.