HP MSR2003 Configuration Manual page 286

routers discover their IPv6 PIM neighbors, maintain IPv6 PIM neighboring relationship with other routers,
and build and maintain SPTs.
SPT building
The process of building an SPT is the flood-and-prune process:
1. In an IPv6 PIM-DM domain, the IPv6 multicast data from the IPv6 multicast source S to the IPv6
multicast group G is flooded throughout the domain. A router performs an RPF check for the IPv6
multicast data. If the check succeeds, the router creates an (S, G) entry and forwards the data to
all downstream nodes in the network. In the flooding process, all the routers in the IPv6 PIM-DM
domain create the (S, G) entry.
2. The nodes without downstream receivers are pruned. A router that has no downstream receivers
multicasts a prune message to all upstream IPv6 PIM routers on the subnet. When an upstream
node receives the prune message, it removes the receiving interface from the (S, G) entry. In this
way, the upstream stream node stops forwarding subsequent packets addressed to that IPv6
multicast group down to this node.
NOTE:
An (S, G) entry contains an IPv6 multicast source address S, an IPv6 multicast group address G, an
outgoing interface list, and an incoming interface.
A prune process is initiated by a leaf router. As shown in
have any downstream receivers initiates a prune process by sending a prune message toward the IPv6
multicast source. This prune process goes on until only necessary branches are left in the IPv6 PIM-DM
domain, and these necessary branches constitute an SPT.
Figure 83 SPT building
Source
Server
The pruned state of a branch has a finite holdtime timer. When the timer expires, IPv6 multicast data is
again forwarded to the pruned branch. The flood-and-prune cycle takes place periodically to maintain
the forwarding branches.
Graft
A previously pruned branch might have new downstream receivers. To reduce the latency for resuming
the forwarding capability of this branch, a graft mechanism is used as follows:
SPT
Prune message
IPv6 multicast packets
Figure 83, the router interface that does not
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Host A
Receiver
Host B
Receiver
Host C