3Com S7902E, Configuration Manual

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SSM Model Implementation in IPv6 PIM
The source-specific multicast (SSM) model and the any-source multicast (ASM) model are two opposite
models. Presently, the ASM model includes the IPv6 PIM-DM and IPv6 PIM-SM modes. The SSM
model can be implemented by leveraging part of the IPv6 PIM-SM technique.
The SSM model provides a solution for source-specific multicast. It maintains the relationships between
hosts and routers through MLDv2. IPv6 PIM-DM implements IPv6 multicast forwarding by building
SPTs rooted at the IPv6 multicast source through the “flood and prune” mechanism. Although an SPT
has the shortest path, it is built in a low efficiency. Therefore the IPv6 PIM-DM mod is not suitable for
large- and medium-sized networks.
In actual application, part of the IPv6 PIM-SM technique is adopted to implement the SSM model. In the
SSM model, receivers know exactly where an IPv6 multicast source is located by means of
advertisements, consultancy, and so on. Therefore, no RP is needed, no RPT is required, and is no
source registration process is needed for the purpose of discovering IPv6 multicast sources in other
IPv6 PIM domains.
Compared with the ASM model, the SSM model only needs the support of MLDv2 and some subsets of
IPv6 PIM-SM. The operation mechanism of the SSM model in an IPv6 PIM domain can be summarized
as follows:
z
Neighbor discovery
z
DR election
z
SPT building
Neighbor discovery
IPv6 PIM-SSM uses the same neighbor discovery mechanism as in IPv6 PIM-SM. Refer to Neighbor
discovery .
DR election
IPv6 PIM-SSM uses the same DR election mechanism as in IPv6 PIM-SM. Refer to DR election .
SPT building
Whether to build an RPT for IPv6 PIM-SM or an SPT for IPv6 PIM-SSM depends on whether the IPv6
multicast group the receiver is to join falls in the IPv6 SSM group range (the IPv6 SSM group range
reserved by IANA is FF3x::/32, where x represents any legal address scope).