640 CHAPTER 47: PIM CONFIGURATION
Figure 195 SPT establishment in PIM-SSM
As shown in Figure 195, Host B and Host C are multicast information receivers.
They send IGMPv3 report messages denoted as (Include S, G) to the respective DRs
to express their interest in the information of the specific multicast source S. If they
need information from other sources than S, they send an (Exclude S, G) report.
No matter what the description is, the position of multicast source S is explicitly
specified for receivers.
The DR that has received the report first checks whether the group address in this
message falls 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 (Include S, G) or (Exclude 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 PIM-SM process is followed: the DR needs to send a (*, G) join
message to the RP, and a multicast source registration process is needed.
n
In PIM-SSM, the “channel” concept is used to refer to a multicast group, and the
“channel subscription” concept is used to refer to a join message.
Protocols and Standards PIM-related specifications are as follows:
■ RFC 2362: Protocol Independent Multicast-sparse Mode (PIM-SM): Protocol
Specification
■ RFC 3973: Protocol Independent Multicast-Dense Mode (PIM-DM): Protocol
Specification(Revised)
■ draft-ietf-pim-sm-v2-new-06: Protocol Independent Multicast-Sparse Mode
(PIM-SM)
■ draft-ietf-pim-dm-new-v2-02: Protocol Independent Multicast-Dense Mode
(PIM-DM)
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
SPT
Subscribe message
RP DR
DR
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