Multicast forwarding follows a tree
PIM is router-to-router multicast signaling. It uses routing information to find the upstream path toward a source or rendezvous point, independent of which unicast routing protocol supplied that information. Reverse path forwarding (RPF) helps keep multicast traffic on the expected tree.
A (*,G) entry represents state for a group G shared by sources. An (S,G) entry represents a specific source S and group G. Host membership on a receiver-facing link can cause a router to join an upstream PIM tree.
Dense, sparse, source-specific and bidirectional trees
Dense mode initially floods traffic along eligible paths and prunes branches without receivers. Sparse mode normally builds a shared tree toward a rendezvous point (RP) for any-source multicast. A source’s first-hop router can register the source with the RP, and a downstream router can later switch to a source-specific shortest path tree.
In source-specific multicast (SSM), a receiver names a source and group. PIM joins directly toward that source, with no RP or shared tree for the channel. Bidirectional PIM instead uses a shared tree that carries traffic both toward and away from the root. It elects a designated forwarder on each link for the rendezvous point address and avoids per-source forwarding state for those groups.
Try it in a lab
- On a lab platform supporting PIM SSM, place a sender and receiver on different routed subnets and enable PIM on the transit path. Have the receiver join 232.1.1.1 specifically from the sender’s IPv4 address.
- Inspect the receiver-facing router’s IGMP membership and the transit routers’ (S,G) state. Confirm that the upstream interface points toward the source and that traffic arrives.
- Send to the same group from a second source without adding it to the subscription. Confirm that the receiver’s selected channel still carries only the first source.