In layman's terms, it is a multicast routing protocol operating between Layer 3 routers.

What problems does it solve?

When transmitting multicast traffic across the network, routers need to identify which downstream neighbors should receive replicated data. PIM exchanges messages between routers to dynamically establish loop-free multicast forwarding paths (distribution trees). It ensures data is precisely delivered to all multicast members while preventing bandwidth waste.

PIM-DM (Dense Mode)

Concept: Assume every subnet has receivers. Traffic is pushed to the entire network first, and unnecessary branches are pruned afterward.

Mechanism: Adopt the "push-and-prune" approach. Multicast traffic is flooded initially. If a receiving router detects no downstream receivers, it sends prune messages to block traffic.

Applicable scenarios: Networks with densely distributed multicast members where nearly all terminals require multicast streams. It is rarely deployed nowadays due to high flooding overhead.

PIM-SM (Sparse Mode)

Concept: Assume receivers are sparse and scattered. No traffic is forwarded by default; forwarding is triggered only upon explicit join requests.

Mechanism: Adopt the "pull" approach. The receiver-side router sends explicit join messages toward a Rendezvous Point (RP). The source sends traffic to the RP via unicast first, and the RP forwards traffic to receivers along the shared tree. The path can later be optimized by switching to the source tree.

Applicable scenarios: The mainstream standard for modern networks. Suitable for most scenarios with sparsely distributed receivers and cross-WAN transmission.