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Figure 40 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.
NOTE:
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 toward its upstream node as
a request to join the SPT again.
2. After 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
Where more than one multicast router exists, the assert mechanism shuts off duplicate multicast flows
onto the same multi-access network. It does this by electing a unique multicast forwarder on the
multi-access network.
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