146
As shown in Figure 49, for the multicast groups in a specific group address range, the
admin-scope zones must be geographically separated and isolated. A router cannot belong to
multiple admin-scope zones. In other words, different admin-scope zones contain different routers.
However, the global-scoped zone includes all routers in the PIM-SM domain or BIDIR-PIM domain.
Multicast packets that do not belong to any admin-scope zones are forwarded in the entire PIM-SM
domain or BIDIR-PIM domain.
• In view of multicast group address ranges
Each admin-scope zone provides services for specific multicast groups, of which the multicast
group addresses are valid only within the local zone. The multicast groups of different
admin-scoped zones might have intersections. The global-scoped zone is designated to all the
multicast groups other than those of the admin-scoped zones.
Figure 50 Relationship in view of multicast group address ranges
In Figure 50, the admin-scoped zones 1 and 2 have no intersection, but the admin-scoped zone 3
is a subset of the admin-scoped zone 1. The global-scoped zone provides services for all the
multicast groups that are not covered by the admin-scoped zones 1 and 2, that is, G−G1−G2 in
this case.
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. It is also called "PIM-SSM."
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 IGMPv3 or 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 is needed, no RPT is required, no source registration process exists, and the multicast source
discovery protocol (MSDP) is not needed for discovering sources in other PIM domains.
In PIM-SSM, the term "channel" refers to a multicast group, and the term "channel subscription" refers to
a join message.
Admin-scope 3
G3 address
Admin-scope 2
G2 address
Admin-scope 1
G1 address
Global-scope
G
−G1−G2 address
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