22. Multicast Advanced

1. 18.7 - PIM-SSM

Source-Specific Multicast removes the Rendezvous Point from the data path and the control plane. A receiver joins (S,G) directly by sending an IGMPv3 INCLUDE report for a known source and group, normally inside 232.0.0.0/8.

2. 18.8 - RPF

Reverse Path Forwarding is multicast loop prevention. The router accepts a multicast packet only when it arrives on the interface the routing table would use to reach the packet source. For (S,G)state the lookup is toward source S; for (*,G) state it is toward the RP.

In production, the RPF route can come from the unicast RIB or from an MRIB tuned for multicast. The MRIB is useful when the best unicast path is asymmetric or when multicast needs a different upstream than normal traffic.

3. 18.9 - AutoRP and BSR

Static RP configuration works in small labs but becomes fragile across a routed domain. AutoRP and BSR distribute group-to-RP mappings so every PIM router can build shared trees consistently.

4. 18.10 - MSDP and Anycast-RP

MSDP connects PIM-SM domains by advertising active sources between RPs. When a source registers with one RP, that RP sends a Source-Active message containing (S,G,RP). A remote RP with interested receivers can then join the source tree toward S.

Anycast-RP uses the same RP address on multiple routers. Unicast routing sends each source or receiver to the nearest RP, while MSDP synchronizes source knowledge between the RP nodes. The group-to-RP address stays stable during RP failure or maintenance.

5. 18.11 - Bidir-PIM

Bidirectional PIM is optimized for many-to-many groups. It keeps only (*,G) state, forwards traffic both toward and away from the RP-rooted shared tree, and elects a Designated Forwarder on each link to decide which router may forward upstream.

The tradeoff is path optimality. Bidir-PIM avoids source registration and (S,G)state explosion, but traffic follows the shared tree rather than switching to a per-source shortest path tree.

6. 18.12 - BIER

Bit Index Explicit Replication changes the multicast scaling problem. The ingress router encodes intended egress routers in a bitstring, and each transit router uses its Bit Index Forwarding Table to copy packets only to next hops whose masks intersect the remaining bitstring.

• Core routers keep BIFT state, not per-group (*,G) or (S,G) state.
• Ingress BFRs own receiver expansion and set bits for the egress BFRs that need the packet.
• BIER-TE adds explicit traffic-engineered paths while preserving bitstring-based replication.

7. 18.13 - Multicast in EVPN-VXLAN

VXLAN fabrics must carry BUM traffic across VTEPs. The simplest model is ingress replication, where the source VTEP sends one VXLAN copy to every remote VTEP in the VNI. Underlay multicast moves replication into the fabric, while EVPN selective multicast advertises receiver interest through BGP.

8. Quiz / Interview Checks