3Com Switch 8800 Advanced Software V5 Configuration Guide

574 CHAPTER 42: PIM CONFIGURATION
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:
■ Neighbor discovery
■ DR election
■ 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 564 described above.
DR election
PIM-SSM uses the same DR election mechanism as in PIM-SM. Refer to “DR
election” on page 567 described above.
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 address range (the
default SSM group address range is 232.0.0.0/8).
Figure 182 SPT establishment in PIM-SSM
As shown in Figure 182, 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.
However described, 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 address range:
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
SPT
Subscribe message
RP DR
DR