What is the main function of the PIM-SM protocol?

The main function of the Protocol Independent Multicast-Sparse Mode (PIM-SM) protocol is to manage multicast distribution. This protocol is specifically designed for efficiently routing multicast traffic, which involves sending a single packet to multiple destinations at once. PIM-SM operates in sparse environments where multicast receivers are not densely located together, thereby optimizing the flow of multicast data by building a shared tree for multicast groups.

PIM-SM utilizes a two-phase approach: first, it establishes a shared tree rooted at a rendezvous point (RP) and then, if desired, can switch to a source-specific shortest path tree (SPT) for more efficient data transmission once traffic patterns are established. By effectively managing how multicast packets are distributed across the network, PIM-SM ensures that multicast transmissions reach all intended recipients without excessive bandwidth usage or unnecessary duplication of packets.

In contrast to the other choices, which address different networking functions, PIM-SM's primary focus lies in managing multicast traffic rather than simply generating trees, ensuring bandwidth, or performing unicast routing.