What is the purpose of a PIM Graft Message?

A PIM Graft message serves the purpose of unpruning a pruned interface. In Protocol Independent Multicast (PIM), when a multicast source sends traffic, it can create a distribution tree to route the multicast packets efficiently to receivers. If certain interfaces were pruned from the multicast distribution tree because there were no receivers present at that time, a Graft message can be used when a new receiver indicates it wants to receive the multicast traffic on that previously pruned interface.

When a receiver joins, it sends a Join message, and if the upstream router has a pruned interface that needs to start receiving the multicast traffic again, it sends a Graft message to unprune that interface. This allows the previously pruned interface to start receiving multicast packets, ensuring the multicast traffic can reach all intended recipients.

Understanding this concept highlights the dynamic nature of multicast routing where interfaces can be pruned or unpruned based on the presence or absence of receivers, thus optimizing network resources and bandwidth usage.