55
Figure 29
SPT building in PIM-SSM
As shown in
, Host B and Host C are multicast information receivers. They send IGMPv3 report
messages to the respective DRs to express their interest in the information about the specific multicast
source S.
After receiving a report message, the DR first examines whether the group address in this message falls
into the SSM group range and does the following:
•
If the group address falls into the SSM group range, 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. An SPT is thereby
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 the group address does not fall into the SSM group range, the receiver-side DR sends a (*, G) join
message to the RP, and the source-side DR registers the multicast source.
In PIM-SSM, the term "channel" refers to a multicast group, and the term "channel subscription" 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. When they run together, which one is chosen for a receiver
trying to join a group depends, as shown in
.
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
