HP A8800 Configuration Manual page 124

Figure 37 SPT establishment
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 a multicast group, to reduce the join latency, PIM-DM uses
a graft mechanism to resume data forwarding to that branch. The process is as follows:
1. The node that needs to receive multicast data sends a graft message hop by hop toward its
upstream node, as a request to join the SPT again.
2. Upon receiving this graft message, the upstream node puts the interface on which the graft was
received into the forwarding state and responds with a graft-ack message to the graft sender.
3. If the node that sent a graft message does not receive a graft-ack message from its upstream node,
it will keep sending graft messages at a configurable interval until it receives an acknowledgment
from its upstream node.
Assert
If multiple multicast routers exist on a multi-access subnet, duplicate packets may flow to the same subnet.
To shut off duplicate flows, the assert mechanism is used for election of a single multicast forwarder on
a multi-access network.
SPT
Prune message
Multicast packets
112
Host A
Receiver
Host B
Receiver
Host C