3Com 4800G Series, Configuration Manual

Page: 639 / 1246

PIM Overview 639
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The global scope zone and each BSR admin-scope region have their own C-RPs
and BSR. These devices are effective only in their respective admin-scope
regions. Namely, the BSR election and RP election are implemented
independently within each admin-scope region.
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Each BSR admin-scope region has its own boundary. The multicast information
(such as C-RP-Adv messages and BSR bootstrap messages) can be transmitted
only within the domain.
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Likewise, the multicast information in the global scope zone cannot enter any
BSR admin-cope region.
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In terms of multicast information propagation, BSR admin-scope regions are
independent of one another and each BSR admin-scope region is independent
of the global scope zone, and no overlapping is allowed between any two BSR
admin-scope regions.
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:
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Neighbor discovery
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DR election
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SPT building
Neighbor discovery
PIM-SSM uses the same neighbor discovery mechanism as in PIM-DM and
PIM-SM. Refer to “Neighbor discovery” on page 630.
DR election
PIM-SSM uses the same DR election mechanism as in PIM-SM. Refer to “DR
election” on page 633.
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).