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SPT building
When a receiver joins an IPv6 multicast group, the receiver-side DR uses the IPv6 SSM group range to
determine whether to build an RPT for IPv6 PIM-SM or an SPT for IPv6 PIM-SSM. The IPv6 SSM group
range reserved by IANA is FF3x::/32, where "x" represents any legal address scope.
Figure 95 SPT building in IPv6 PIM-SSM
As shown in Figure 95, 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. The following takes one DR as an example:
After receiving a report message, the DR first checks whether the group addresses 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 send a subscribe message toward the
IPv6 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 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 DRs send a (*, G) join
message to the RP. The IPv6 multicast source registers with the RP.
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 96 show
s
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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