Physical layer problems cause the most frustration. Everything looks configured correctly but nothing works. Always check Layer first.
Cable issues:
- Loose connections at either end
- Damaged cables from bending or pinching
- Wrong cable type for the distance
Port and hardware:
- Link lights off or amber instead of green
- Speed or duplex mismatch between devices
- Failed port or NIC
Quick checks:
- Swap cables with known-good ones
- Try a different port on the switch
- Check link lights on both ends
- Look for physical damage