An interface can show up as ifindex0 for a variety of reasons
- When no input I/F exists for the packet, ifindex is set to zero. Example: The packet is generated on the router
- When no output I/F exists for the packet, ifindex is set to zero. Example: The destination of the packet is the router itself
- The packet is dropped on the router
- Multicast traffic
- Conversation denied by ACL rule
- Conversation dropped by QoS
- Router misconfiguration or bug
