Initiating PIM multicasts on a network

Once PIM is enabled on each device, a network user can begin a video conference multicast from the server on R1 as shown in Figure 3. When a multicast packet is received on a PIM-capable device interface, the interface checks its IP routing table to determine whether the interface that received the message provides the shortest path back to the source. If the interface does provide the shortest path back to the source, the multicast packet is then forwarded to all neighboring PIM devices. Otherwise, the multicast packet is discarded and a prune message is sent back upstream.

In Figure 3, the root node (R1) is forwarding multicast packets for group 229.225.0.1, which it receives from the server, to its downstream nodes, R2, R3, and R4. Device R4 is an intermediate device with R5 and R6 as its downstream devices. Because R5 and R6 have no downstream interfaces, they are leaf nodes. The receivers in this example are those workstations that are resident on devices R2, R3, and R6.