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1.2.2. The Mental Model for Troubleshooting
When something breaks, the layered model tells you where to look. Start at Layer 1—if bits can't traverse the wire, nothing else matters:
| Order | Layer | Check | Tools | Symptoms if Broken |
|---|---|---|---|---|
| 1st | Physical | Cables, lights, connections | Cable tester, visual | No link light, "cable unplugged" |
| 2nd | Data Link | MAC addresses, switch ports | show mac-address-table | No local connectivity |
| 3rd | Network | IP addresses, routing | ping, traceroute | Can't reach remote networks |
| 4th | Transport | Port connectivity, firewalls | netstat, telnet [ip] [port] | Can ping but can't connect |
| 5th-7th | Application | Service configuration | Application logs | Service-specific errors |
⚠️ Exam Trap: Questions test whether you can identify which layer a problem exists at. A "no link light" is Layer 1. A "duplicate IP" is Layer 3. A "can ping but can't browse" suggests Layer 4-7 (likely firewall or DNS). The trap is jumping to complex explanations when simple layer-by-layer diagnosis points to the answer.
Written byAlvin Varughese
Founder•15 professional certifications