3Com Switch 4800G 24-Port Configuration Manual page 631

A prune process is first initiated by a leaf router. As shown in Figure 187, a router
without any receiver attached to it (the router connected with Host A, for
example) sends a prune message, and this prune process goes on until only
necessary branches are left in the PIM-DM domain. These branches constitute the
SPT.
Figure 187 SPT establishment
Source
Server
SPT
Prune message
Multicast packets
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.
n
Pruning has a similar implementation in PIM-SM.
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 the source, 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.
PIM Overview 631
Host A
Receiver
Host B
Receiver
Host C