3Com S7906E - Switch, Configuration Manual

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1-4
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.
Figure 1-2 Assert mechanism
As shown in Figure 1-2 , after Router A and Router B receive an (S, G) IPv6 multicast packet from the
upstream node, they both forward the packet to the local subnet. As a result, the downstream node
Router C receives two identical multicast packets, and both Router A and Router B, on their own local
interface, receive a duplicate IPv6 multicast packet forwarded by the other. Upon detecting this
condition, both routers send an assert message to all IPv6 PIM routers through the interface on which
the packet was received. The assert message contains the following information: the multicast source
address (S), the multicast group address (G), and the preference and metric of the IPv6 unicast route to
the source. By comparing these parameters, either Router A or Router B becomes the unique forwarder