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Figure 50 SPT building in PIM-SSM
As shown in
, Host B and Host C are receivers. They send IGMPv3 report messages to
their DRs to express their interest in the multicast information that the multicast source S sends to
the multicast group G.
After receiving a report message, the DR first checks whether the group address in this message is
in the SSM group range and does the following:
•
If the group address is in the SSM group range, the DR sends a subscribe message hop by hop
toward the multicast source S. All routers along the path from the DR to the source create an (S,
G) entry to build an SPT. The SPT is rooted at the multicast source S and has the receivers as
its leaves. This SPT is the transmission channel in PIM-SSM.
•
If the group address is not in the SSM group range, the receiver-side DR sends a (*, G) join
message to the RP. The multicast source registers with the source-side DR.
In PIM-SSM, the term "subscribe message"
refers to a join message.
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.
shows how the device selects one
protocol from among them for a receiver trying to join a group.
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