HP 5920 series Configuration Manual page 69

Graft
To reduce the join latency when a new receiver on a previously pruned branch joins a multicast group,
PIM-DM uses a graft mechanism to turn the pruned branch into a forwarding branch, as follows:
The node that needs to receive the multicast data sends a graft message to its upstream node,
1.
telling it to rejoin the SPT.
After receiving this graft message, the upstream node adds the interface that received the graft
2.
message into the outgoing interface list of the (S, G) entry for the multicast group, and then sends
a graft-ack message to the graft sender.
If the node that sent a graft message does not receive a graft-ack message from its upstream node,
3.
it 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 25 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, and both 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
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:
The router with a higher preference to the multicast source wins.
1.
If both routers have the same preference to the source, the router with a smaller metric to the
2.
multicast source wins.
If a tie exists in route metric to the multicast source, the router with a higher IP address on the
3.
downstream interface wins.
Figure 25, after Router A and Router B receive an (S, G) packet from the upstream node, they
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