339
SPT building
The decision to build an RPT for IPv6 PIM-SM or an SPT for IPv6 PIM-SSM depends on whether the
IPv6 multicast group that the receiver host joins is in the IPv6 SSM group range. The IPv6 SSM
group range reserved by IANA is FF3x::/32, where "x" represents any legal address scope.
Figure 103 SPT building in IPv6 PIM-SSM
As shown in Figure 103, Host B and Host C are receivers. They send MLDv2 report messages to
their DRs to express their interest in the multicast information that the IPv6 multicast source S sends
to the IPv6 multicast group G.
After receiving a report message, the DR first checks whether the group address in the message is in
the IPv6 SSM group range and does the following:
If the group address is in the IPv6 SSM group range, the DR sends a subscribe message hop
by hop toward the IPv6 multicast source S. All routers along the path from the DR to the IPv6
multicast source create an (S, G) entry to build an SPT. The SPT is rooted the IPv6 multicast
source S and has the receivers as its leaves. This SPT is the transmission channel in IPv6
PIM-SSM.
If the group address is not in the IPv6 SSM group range, the receiver-side DR sends a (*, G) join
message to the RP. The IPv6 multicast source registers with the source-side DR.
In IPv6 PIM-SSM, the term "subscribe message" refers to a join message.
Relationship among IPv6 PIM protocols
In an IPv6 PIM network, IPv6 PIM-DM cannot run together with IPv6 PIM-SM, IPv6 BIDIR-PIM, or
IPv6 PIM-SSM. However, IPv6 PIM-SM, IPv6 BIDIR-PIM, and IPv6 PIM-SSM can run together.
Figure 104 shows how the device selects one protocol from among them for a receiver trying to join
a group.
For more information about MLD SSM mapping, see "Configuring MLD."
Source
Server
Host A
Host B
Host C
Receiver
Receiver
IPv6 multicast packets
SPT
Subscribe message
DR
DR
RP
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