H3C S5500-EI Configuration Manual page 387

For a given IPv6 multicast stream, the interface that receives the IPv6 multicast stream is referred to
as "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
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 92 SPT establishment in an IPv6 PIM-DM domain
Source
Server
The flood-and-prune process takes place periodically. A pruned state timeout mechanism is provided. A
pruned branch restarts multicast forwarding when the pruned state times out and then is pruned again
when it no longer has any multicast receiver.
Graft
When a host attached to a pruned node joins an IPv6 multicast group, to reduce the join latency, IPv6
PIM-DM uses the graft mechanism to resume IPv6 multicast data forwarding to that branch. The process
is as follows:
The node that needs to receive IPv6 multicast data sends a graft message toward its upstream node
1.
as a request to join the SPT again.
After receiving this graft message, the upstream node puts the interface on which the graft was
2.
received into the forwarding state and responds with a graft-ack message to the graft sender.
If the node that sent a graft message does not receive a graft-ack message from its upstream node,
3.
it keeps sending graft messages at a configurable interval until it receives an acknowledgment
from its upstream node.
SPT
Prune message
IPv6 multicast packets
Figure 92, a router without any receiver attached
368
Host A
Receiver
Host B
Receiver
Host C