H3C S7500 Series Operation Manual page 477

Operation Manual – Multicast
H3C S7500 Series Ethernet Switches
Source
Server
Figure 7-1 Diagram for SPT establishment in PIM-DM
The process above is called "Flooding and Pruning". Every pruned node also provides
a timeout mechanism. If the pruning behavior times out, the router will initiate another
flooding and pruning process. This process is performed periodically for PIM-DM.
III. Graft
When a pruned downstream node needs to resume the forwarding state, it will send a
graft message to inform the upstream node. As shown in
receive multicast data again, graft messages will be sent hop by hop to the multicast
source S. The intermediate nodes will return acknowledgements when receiving Graft
messages. Thus, the pruned branches are restored to the information transmission
state.
IV. RPF check
PIM-DM adopts the RPF check mechanism to establish a multicast forwarding tree
from the data source S based on the existing unicast routing table, static multicast
routing table, and MBGP routing table.
The procedure is as follows:
When a multicast packet arrives, the router first checks its path.
If the interface this packet reaches is the one along the unicast route towards the
multicast source, the path is considered as correct.
Otherwise, the multicast packet will be discarded as a redundant one.
The unicast routing information on which the RPF check is based can be of any unicast
routing protocol such as RIP or OSPF. It is independent of the specified unicast routing
protocol. The static multicast routing table needs to be configured manually, and the
MBGP routing table is provided by the MBGP protocol.
SPT
Prune message
Multicast packets
7-3
Chapter 7 PIM Configuration
Host A
Receiver
Host B
Receiver
Host C
Figure 7-1, if user A wants to