01 · Start here
The physical layer carries the signal
Cabling and transceivers form the physical path between network interfaces. Before troubleshooting VLANs or routing, prove that the physical medium, optics and interfaces can carry a clean signal.
Endpoint / Switch
→NIC / Port
→Transceiver
→Cable / Fiber
→Transceiver
→Switch / Router
Engineer takeaway
A Layer-3 symptom can begin with a Layer-1 problem. Always prove the physical path first.