H3C Technologies H3C S7500E Series Switches - Manual (Page 355)

13-4
Figure 13-1 SPT establishment in an IPv6 PIM-DM domain
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 IPv6 PIM-SM.
Graft
When a host attached to a pruned node joins an IPv6 multicast group, to reduce the join latency,
IPv6 PIM-DM uses the graft mechanism to resume IPv6 multicast data forwarding to that
branch. The process is as follows:
1) The node that needs to receive IPv6 multicast data sends a graft message toward 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 shutoff duplicate IPv6 multicast flows onto the same
multi-access network, where more than one multicast routers exists, by electing a unique IPv6
multicast forwarder on the multi-access network.