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Figure 46 Relationship between admin-scope zones and the global-scope zone in group address ranges
In Figure 46, the group address ranges of admin-scope 1 and 2 have no intersection, whereas the
group address range of admin-scope 3 is a subset of the address range of admin-scope 1. The group
address range of the global-scope zone—G-G1-G2—covers all the group addresses other than those of
all the admin-scope zones. A supplementary relationship exists between the global-scope zone and all
the admin-scope zones in terms of group address ranges.
PIM-SSM overview
The source-specific multicast (SSM) model and the any-source multicast (ASM) model are opposites.
Presently, the ASM model includes the PIM-DM and PIM-SM modes. You can implement the SSM model
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 locate a multicast source by means of advertisements, consultancy, and so on. No RP
or RPT is required, no source registration process exists, and no need exists to use the multicast source
discovery protocol (MSDP) for discovering sources in other PIM domains.
The working mechanism of PIM-SSM can be summarized as follows:
• Neighbor discovery
• DR election
• SPT building
Neighbor discovery
PIM-SSM uses the same neighbor discovery mechanism as in PIM-DM and PIM-SM. See “Neighbor
discovery.”
DR election
PIM-SSM uses the same DR election mechanism as in PIM-SM. See “DR election.”
Construction of SPT
The decision to build an RPT for PIM-SM or an SPT for PIM-SSM depends on whether the multicast group
the receiver will join falls into the SSM group range—SSM group range reserved by IANA is
232.0.0.0/8.
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