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 figure below, 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 figure below, 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 sends a prune message to R1. With R4 in a prune state, the resulting multicast delivery tree would consist only of leaf nodes R2 and R3.