Initiating PIM multicasts on a network
PIM was introduced to simplify some of the complexity of the routing protocol at the cost of additional overhead tied with a greater replication of forwarded multicast packets. PIM builds
There are two modes in which PIM operates: Dense and Sparse. The Dense Mode is suitable for densely populated multicast groups, primarily in the LAN environment. The Sparse Mode is suitable for sparsely populated multicast groups with the focus on WAN.
PIM uses the IP routing table instead of maintaining its own, thereby being routing protocol independent.
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 on page 83. 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 on page 83, 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.
Pruning a multicast tree
As multicast packets reach these leaf devices, the devices check their IGMP databases for the group. If the group is not in the IGMP database of the device, the device discards the packet and sends a prune message to the upstream device. The device that discarded the packet also maintains the prune state for the source, group (S,G) pair. The branch is then pruned (removed) from the multicast tree. No further multicast packets for that specific (S,G) pair will be received from that upstream device until the prune state expires. You can configure the PIM Prune Timer (the length of time that a prune state is considered valid).
For example, in the “Transmission of multicast packets from the source to host group members” figure, the sender with address 207.95.5.1 is sending multicast packets to the group 229.225.0.1. If a PIM device receives any groups other than that group, the device discards the group and sends a prune message to the upstream PIM device.
In the “Pruning leaf nodes from a multicast tree” figure, device R5 is a leaf node with no group members in its IGMP database. Therefore, the device must be pruned from the multicast tree. R5 sends a prune message upstream to its neighbor device R4 to remove itself from the multicast delivery tree and install a prune state, as seen in the “Pruning leaf nodes from a multicast tree” figure. Device 5 will not receive any further multicast traffic until the prune age interval expires.
When a node on the multicast delivery tree has all of its downstream branches (downstream interfaces) in the prune state, a prune message is sent upstream. In the case of R4, if both R5 and R6 are in a prune state at the same time, R4 becomes a leaf node with no downstream interfaces and
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