HP 6125XLG Ip Multicast Configuration Manual page 286

Graft
A previously pruned branch might have new downstream receivers. To reduce the latency for resuming
the forwarding capability of this branch, a graft mechanism is used as follows:
1. The node that needs to receive the IPv6 multicast data sends a graft message to its upstream node,
telling it to rejoin the SPT.
2. After receiving this graft message, the upstream node adds the interface that received the graft
message to the outgoing interface list of the (S, G) entry. It also sends a graft-ack message to the
graft sender.
3. If the graft sender receives a graft-ack message, the graft process finishes. Otherwise, the graft
sender keeps sending graft messages at a configurable interval until it receives an
acknowledgment from its upstream node.
Assert
On a subnet with more than one multicast router, the assert mechanism shuts off duplicate multicast flows
to the network. It does this by electing a unique multicast forwarder for the subnet.
Figure 86 Assert mechanism
As shown in
both forward the packet to the local subnet. As a result, the downstream node Router C receives two
identical multicast packets. Both of Router A and Router B, on their own downstream interfaces, receive
a duplicate packet forwarded by the other. After detecting this condition, both routers send an assert
message to all IPv6 PIM routers on the local subnet through the interface that received the packet. The
assert message contains the IPv6 multicast source address (S), the IPv6 multicast group address (G), and
the metric preference and metric of the IPv6 unicast route to the IPv6 multicast source. By comparing these
parameters, either Router A or Router B becomes the unique forwarder of the subsequent (S, G) packets
on the subnet. The comparison process is as follows:
1. The router with a higher metric preference to the IPv6 multicast source wins.
2. If both routers have the same metric preference to the IPv6 multicast source, the router with a
smaller metric wins.
3. If both routers have the same metric, the router with a higher IPv6 link-local address on the
downstream interface wins.
Figure 86, after Router A and Router B receive an (S, G) packet from the upstream node, they
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