H3C Technologies H3C S12500 Series Switches - Manual (Page 127)

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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
The assert mechanism shuts off duplicate multicast flows onto the same multi-access network, where more
than one multicast router exists, by electing a unique multicast forwarder on the multi-access network.
Figure 38 Assert mechanism
As shown in Figure 38 , after Router A and Router B receive an (S, G) packet from the upstream node, they
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/MBGP route/multicast static route to the source. By comparing
these parameters, either Router A or Router B becomes the unique forwarder of the subsequent (S, G)
packets on the multi-access subnet. The comparison process is as follows:
1. The router with a higher preference to the source wins.
2. If both routers have the same preference to the source, the router with a smaller metric to the source
wins.
3. If a tie exists in route metric to the source, the router with a higher IP address on the downstream
interface wins.