3Com SuperStack 4 Configuration Manual page 175

PIM-DM Overview 187
Neighbor discovery
The PIM-DM router uses Hello messages to perform neighbor discovery when it is
started. All network nodes running PIM-DM stay in touch with one another by
periodically sending Hello messages.
Flood&Prune
PIM-DM assumes that all hosts on the network are ready to receive multicast data.
When a multicast source "S" begins to send data to a multicast group "G", after
the router receives the multicast packets, the router will perform RPF check
according to the unicast routing table first. If the RPF check is passed, the router
will create an (S, G) entry and then flood the data to all downstream PIM-DM
nodes. If the RPF check is not passed, that is, multicast packets enter from an error
interface, the packets will be discarded. After this process, an (S, G) entry will be
created in the PIM-DM multicast domain.
If the downstream node has no multicast group members, it will send a Prune
message to the upstream nodes to inform the upstream node not to forward data
to the downstream node. Receiving the prune message, the upstream node will
remove the corresponding interface from the outgoing interface list corresponding
to the multicast forwarding entry (S, G). In this way, a SPT (Shortest Path Tree)
rooted at Source S is built. The pruning process is initiated by leaf routers first.
This process is called "flood & prune" process. In addition, nodes that are pruned
provide timeout mechanism. Each router re-starts the "flood & prune" process
upon pruning timeout. The consistent "flood & prune" process of PIM-DM is
performed periodically.
During this process, PIM-DM uses the RPF check and the existing unicast routing
table to build a multicast forwarding tree rooted at the data source. When a
packet arrives, the router will first judge the correctness of the path. If the
interface that the packet arrives is the one indicated by the unicast routing to the
multicast source, the packet is regarded to be from the correct path. Otherwise,
the packet will be discarded as a redundancy packet without the multicast
forwarding. The unicast routing information as path judgment can come from any
unicast routing protocol independent of any specified unicast routing protocol
such as the routing information learned by RIP and OSPF
Assert Mechanism
As shown in the Figure 43, both routers A and B on the LAN have their own
receiving paths to multicast source S. In this case, when they receive a multicast
packet sent from multicast source S, they will both forward the packet to the LAN.
Multicast Router C at the downstream node will receive two copies of the same
multicast packet.