HP MSR ASM Configuration Manual page 55

The flood-and-prune process takes place periodically. A pruned state timeout mechanism is provided. A
pruned branch restarts multicast forwarding when the pruned state times out and then is pruned again
when it no longer has any multicast receiver.
Graft
When a host attached to a pruned node joins a multicast group, to reduce the join latency, PIM-DM uses
a graft mechanism to resume data forwarding to that branch. The process is as follows:
1. The node that needs to receive multicast data sends a graft message toward its upstream node, as
a request to join the SPT again.
2. After 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
On a shared-media network 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 on the shared-media
network.
Figure 19 Assert mechanism
As shown in
1. After Router A and Router B receive an (S, G) packet from the upstream node, both routers 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.
2. 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/MBGP route/multicast static route to the source.
3. The routers compare these parameters, and either Router A or Router B becomes the unique
forwarder of the subsequent (S, G) packets on the shared-media subnet. The comparison process
is as follows:
a. The router with a higher preference to the source wins.
Figure 19, the assert mechanism is as follows:
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