H3C Technologies H3C S12500 Series Switches - Manual (Page 139)

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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 will join is in the SSM group range. (SSM group range reserved by IANA is 232.0.0.0/8.)
Figure 48 SPT building in PIM-SSM
As shown in Figure 48 , Host B and Host C are multicast information receivers. They send IGMPv3 report
messages to the respective DRs to express their interest in a specific multicast source S.
After receiving a report message, the DR first determines whether the group address in this message is in
the SSM group range:
• If so, the DR sends a subscribe message for channel subscription hop by hop toward the multicast
source S. An (S, G) entry is created on all routers on the path from the DR to the source. Thus, an SPT
is built in the network, with the source S as its root and receivers as its leaves. This SPT is the
transmission channel in PIM-SSM.
• If not, the DR follows the PIM-SM process. The receiver-side DR sends a (*, G) join message to the
RP, and the source-side DR starts a multicast source registration process.
Relationship among PIM protocols
In a PIM network, PIM-DM cannot run together with PIM-SM, BIDIR-PIM, or PIM-SSM. However, PIM-SM,
BIDIR-PIM, and PIM-SSM can run together. When they run together, which one is chosen for a receiver
trying to join a group depends, as shown in Figure 49 .
For more information about IGMP SSM mapping, see "Configuring IGMP."
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
SPT
Subscribe message
DR
DR
RP