Operation Manual – Multicast ProtocolH3C S3610&S5510 Series Ethernet Switches Chapter 6 PIM Configuration6-4Figure 6-1 SPT establishmentThe “flood and prune” process takes place periodically. A pruned state timeoutmechanism is provided. A pruned branch restarts multicast forwarding when the prunedstate 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. GraftWhen a host attached to a pruned node joins a multicast group, to reduce the joinlatency, 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 hoptoward the source, as a request to join the SPT again.2) Upon receiving this graft message, the upstream node puts the interface on whichthe graft was received into the forwarding state and responds with a graft-ackmessage to the graft sender.3) If the node that sent a graft message does not receive a graft-ack message fromits upstream node, it will keep sending graft messages at a configurable intervaluntil it receives an acknowledgment from its upstream node.
