3Com Switch 8800 Advanced Software V5 Configuration Guide

674 CHAPTER 46: IPV6 PIM CONFIGURATION
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 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.
Assert
If multiple multicast routers exist on a multi-access subnet, duplicate IPv6 multicast
packets may flow to the same subnet. To shutoff duplicate flows, the assert
mechanism is used for election of a single IPv6 multicast forwarder on a
multi-access network.
Figure 202 Assert mechanism
As shown in Figure 202, 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 of the subsequent (S, G) IPv6 multicast packets on the multi-access
subnet. The comparison process is as follows:
1 The router with a higher IPv6 unicast route preference to the source wins;
2 If both routers have the same IPv6 unicast route preference to the source, the
router with a smaller metric to the source wins;
Ethernet
Router A Router B
Router C Receiver
IPv6 multicast packets
Assert message
Assert message