H3C Technologies H3C SecPath F1000-E - Manual (Page 99)

3
A prune process is first initiated by a leaf router. As shown in Figure 1 , 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 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.
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 towards 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
The assert mechanism is used to shut off duplicate multicast flows onto the same multi-access network,
where more than one multicast router exists, by electing a unique multicast forwarder on the multi-access
network.