3Com S7902E, Configuration Manual

Page: 1035 / 2621

1-13
SSM Model Implementation in PIM
The source-specific multicast (SSM) model and the any-source multicast (ASM) model are two opposite
models. Presently, the ASM model includes the PIM-DM and PIM-SM modes. The SSM model can be
implemented by leveraging part of the PIM-SM technique.
The SSM model provides a solution for source-specific multicast. It maintains the relationships between
hosts and routers through IGMPv3.
In actual application, part of the PIM-SM technique is adopted to implement the SSM model. In the SSM
model, receivers know exactly where a multicast source is located by means of advertisements,
consultancy, and so on. Therefore, no RP is needed, no RPT is required, there is no source registration
process, and there is no need of using the multicast source discovery protocol (MSDP) for discovering
sources in other PIM domains.
Compared with the ASM model, the SSM model only needs the support of IGMPv3 and some subsets
of PIM-SM. The operation mechanism of PIM-SSM can be summarized as follows:
z
Neighbor discovery
z
DR election
z
SPT building
Neighbor discovery
PIM-SSM uses the same neighbor discovery mechanism as in PIM-DM and PIM-SM. Refer to Neighbor
discovery .
DR election
PIM-SSM uses the same DR election mechanism as in PIM-SM. Refer to DR election .
Construction of SPT
Whether to build an RPT for PIM-SM or an SPT for PIM-SSM depends on whether the multicast group
the receiver is to join falls in the SSM group range (SSM group range reserved by IANA is 232.0.0.0/8).
Figure 1-9 SPT establishment in PIM-SSM