H3C S5500-EI Series Operation Manual page 113

Operation Manual – Multicast
H3C S5500-EI Series Ethernet Switches
Figure 7-1 SPT establishment
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.
III. 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.
7-4
Chapter 7 PIM Configuration