HP 6125XLG Ip Multicast Configuration Manual page 94

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 multicast data sends a graft message to its upstream node,
telling it to rejoin the SPT.
2. After receiving this graft message from an interface, the upstream node adds the receiving
interface 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 continues to send 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 33 Assert mechanism
As shown in
they both forward the packet to the local subnet. As a result, the downstream node Router C receives two
identical multicast packets. 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 PIM routers (224.0.0.13) on the local subnet through the interface that received the packet.
The assert message contains the multicast source address (S), the multicast group address (G), and the
metric preference and metric of the unicast route/static multicast route to the multicast source. By
comparing these parameters, either Router A or Router B becomes the unique forwarder of the
subsequent (S, G) packets on the shared-media LAN. The comparison process is as follows:
1. The router with a higher metric preference to the multicast source wins.
2. If both routers have the same metric preference to the source, the router with a smaller metric wins.
3. If both the routers have the same metric, the router with a higher IP address on the downstream
interface wins.
Figure 33, when Router A and Router B receive an (S, G) packet from the upstream node,
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