Multicast Quick Reference: RPF, PIM-SM, IGMP, SSM, Bidir-PIM & MSDP

RPF checks the packet's SOURCE address (not the group address) against the unicast routing table, confirming traffic arrived on the interface that leads back toward the source. A mismatch drops the packet — this is what prevents multicast forwarding loops. Because RPF depends on unicast routing, an unrelated unicast routing change can break a previously working multicast stream.

The RP is the meeting point: sources register with it, receivers join toward it, building a shared (*, G) tree. Once real traffic flows, a receiver's last-hop router can switch to a direct (S, G) shortest-path tree, bypassing the RP for the data itself. Every router in the domain must agree on the same RP address.

IGMPv2: any-source join — 'give me this group, from anyone.' IGMPv3: source-specific INCLUDE/EXCLUDE reports — 'give me this group, only from this source' (or all except it). IGMPv3 is a hard prerequisite for genuine SSM — without it, there's no source-specific signal for the network to act on.

SSM (default range 232.0.0.0/8): no RP at all — a direct (S, G) tree is built the moment a receiver's IGMPv3 join names a source. Trades RP-related complexity for requiring receivers to already know their source.

Bidir-PIM: never builds (S, G) state — only shared (*, G), used bidirectionally. Purpose-built for many-source, many-to-many traffic patterns where PIM-SM's per-source state would explode. Uses a Designated Forwarder (not a DR) to prevent duplicate forwarding on shared segments.

MSDP: lets RPs in separate PIM-SM domains exchange Source Active messages, enabling cross-domain source discovery. Doesn't apply to SSM (no RP-based discovery needed) or typical Bidir-PIM (no per-source state to announce).