2.4. Interface and Cable Issues

💡 First Principle: Interface problems fall into two categories: physical layer issues (cables, connectors, optics) and configuration mismatches (speed, duplex). Think of it like car trouble: the symptoms might look similar, but you wouldn't replace the engine for a flat tire. The show interfaces command reveals which category you're dealing with.

Imagine this troubleshooting scenario: A user reports their network is "slow." You check their switchport and see: CRC: 2847, late collisions: 245. Your instinct says "bad cable"—you swap it. No improvement. You swap the patch panel port. No improvement. Finally you notice the port is set to full-duplex while the user's NIC is stuck at half-duplex. Hours wasted because you didn't read the counters correctly.

What happens when you misdiagnose: Physical problems require hardware replacement. Configuration problems require settings changes. Replacing hardware for a configuration problem wastes time and money. Changing settings for a physical problem does nothing. The counters tell you which direction to go.

Common Issues and Indicators:

Issue	Symptoms	`show interfaces` Evidence
Collisions	Shared media contention	High collision counter
Late collisions	Cable too long or duplex mismatch	Late collision counter > 0
CRC errors	Bad cable, interference, bad NIC	CRC errors, input errors
Duplex mismatch	One side full, other half	High collisions + CRC errors
Speed mismatch	Interface down or errors	Interface may show down

Duplex Mismatch Scenario: When one side is set to full duplex and the other to half duplex, the full-duplex side doesn't detect collisions (because it doesn't expect them), leading to late collisions and CRC errors.

Switch# show interfaces gigabitEthernet 0/1
  Input errors: 153, CRC: 127, frame: 0
  Output errors: 0, collisions: 2847, late collision: 245

⚠️ Exam Trap: If you see late collisions on a switched network (not a hub), suspect duplex mismatch. Normal collisions shouldn't occur on a full-duplex switched port.