3Com Switch 7750 Configuration Guide

PIM Overview 463
Figure 112 Diagram for SPT establishment in PIM-DM
The process above is called “Flooding and Pruning”. Every pruned node also
provides timeout mechanism. If pruning behavior times out, the router will initiate
another flooding and pruning process. This process is performed periodically for
PIM-DM.
Graft
When a pruned downstream node needs to be restored to the forwarding state, it
may send a graft packet to inform the upstream node. As shown in Figure 113,
user A receives 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.
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 the 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 path judgment 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.
n
When multiple equivalent routes exit, the RPF check mechanism selects the
upstream interface with the highest IP address as the incoming interface for the
packet.
Source
Server
Host A
Host B
Host C
Receiver
Receiver
Multicast packets
SPT
Prune message