4000 Router Series IP Multicast Configuration Guide

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, multicast

sources do not register, and the MSDP is not needed for discovering multicast sources in other PIM

domains.

The operating mechanism of PIM-SSM is summarized as follows:

• Neighbor discovery

• DR election

• SPT building

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 receiver wants to join is included in the SSM group range (232.0.0.0/8 reserved by IANA).

Figure 32 SPT building in PIM-SSM

As shown in Figure 32, 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

Source

Server

Host A

Host B

Host C

Receiver

Multicast packets

SPT

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