3Com 5500-EI PWR Install Manual page 370

corresponding interface from the outgoing interface list in the (S, G) entry and stop forwarding
subsequent packets addressed to that multicast group down to this node.
An (S, G) entry contains the multicast source address S, multicast group address G, outgoing
interface list, and incoming interface.
For a given multicast stream, the interface that receives the multicast stream is referred to as
"upstream", and the interfaces that forward the multicast stream are referred to as "downstream".
A prune process is first initiated by a leaf router. As shown in
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 4-1 SPT building
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.
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:
Figure 4-1, a router without any receiver
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