A stormwater bureau sees multicast video arrive at a router but get discarded before replication. Operators notice the source’s unicast best path is a different interface than the one that received the packet. What control-plane check most likely caused the drop?
Select an answer to reveal the explanation.
Short Explanation
Multicast routers are picky about which door a packet walked through. They check that the arrival interface matches the path they’d use to reach the source—if it doesn’t, RPF fails and the packet is tossed. Wrong incoming interface is the classic silent multicast killer.
Full Explanation
PIM Reverse Path Forwarding verifies that a multicast packet arrives on the interface toward the source (or RP, depending on tree type) per the unicast RIB (or a dedicated multicast RPF table). Arrival on any other interface fails RPF and the packet is dropped to prevent loops. Asymmetric routing, missing routes, or incorrect unicast metrics commonly trigger RPF failures in civic multicast deployments.