HP A5500 SI Switch Series Configuration Manual page 340

When a new receiver on a previously pruned branch joins an IPv6 multicast group, to reduce the
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join latency, IPv6 PIM-DM uses the graft mechanism to resume IPv6 multicast data forwarding to
that branch.
In general, the IPv6 multicast forwarding path is a source tree. That is, it is a forwarding tree with the
IPv6 multicast source as its "root" and IPv6 multicast group members as its "leaves." Because the source
tree is the shortest path from the IPv6 multicast source to the receivers, it is also called shortest path tree
(SPT).
The working mechanism of IPv6 PIM-DM is summarized as follows:
Neighbor discovery
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SPT establishment
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Graft
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Assert
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Neighbor discovery
In an IPv6 PIM domain, a PIM router discovers IPv6 PIM neighbors, maintains IPv6 PIM neighboring
relationships with other routers, and builds and maintains SPTs by periodically multicasting IPv6 PIM
hello messages to all other IPv6 PIM routers on the local subnet.
NOTE:
Every IPv6 PIM enabled interface on a router sends hello messages periodically and, therefore, learns
the IPv6 PIM neighboring information pertinent to the interface.
SPT establishment
The process of constructing an SPT is the flood-and-prune process.
In an IPv6 PIM-DM domain, an IPv6 multicast source first floods IPv6 multicast packets when it
1.
sends IPv6 multicast data to IPv6 multicast group G. The packet undergoes an RPF check. If the
packet passes the RPF check, the router creates an (S, G) entry and forwards the packet to all
downstream nodes in the network. In the flooding process, an (S, G) entry is created on all the
routers in the IPv6 PIM-DM domain.
Nodes without downstream receivers are pruned. A router that has no downstream receivers sends
2.
a prune message to the upstream node to notify the upstream node to delete the corresponding
interface from the outgoing interface list in the (S, G) entry and stop forwarding subsequent packets
addressed to that IPv6 multicast group down to this node.
NOTE:
An (S, G) entry contains the multicast source address S, IPv6 multicast group address G, outgoing
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interface list, and incoming interface.
For a given IPv6 multicast stream, the interface that receives the IPv6 multicast stream is referred to as
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"upstream," and the interfaces that forward the IPv6 multicast stream are referred to as
"downstream."
A leaf router first initiates a prune process. As shown in
attached to it—the router connected with Host A, for example—sends a prune message, and this prune
process continues until only necessary branches remain in the IPv6 PIM-DM domain. These branches
constitute the SPT.
Figure
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86, a router without any receiver