H3C Technologies H3C S7500E Series Switches - Manual (Page 364)

13-13
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).