Master Network
Troubleshooting

5.1

Troubleshooting Methodology

The CompTIA Network+ exam emphasizes a structured 7-step troubleshooting process to approach network issues methodically:

1

Identify the problem

Gather information. "What changed?" Question users (tactfully). Identify symptoms vs. problems. Duplicate the issue if safe.

2

Establish a theory

Question the obvious first. Top-to-bottom or Bottom-up (OSI). Divide and conquer. Don't jump to complex conclusions.

3

Test the theory

Confirm the root cause. If theory is confirmed -> Move to Plan. If NOT confirmed -> Establish new theory or escalate.

4

Establish a plan of action

How will you fix it? Identify potential side effects (will rebooting the router kill the CEO's call?).

5

Implement the solution

Apply the fix. If it's complex, do it during a maintenance window. Escalate if you lack permissions/skill.

6

Verify functionality

Is it REALLY fixed? Have the user confirm. Implement preventive measures to stop recurrence.

7

Document findings

Record symptoms, cause, and solution. Build your Knowledge Base. "If you didn't document it, you didn't do it."

🧠 Quick Check: Methodology

Which step involves creating a Plan of Action?

Step 2: Establish a Theory Step 4: Establish a Plan of Action Step 6: Verify Functionality

Check Answer

5.2

Cabling and Physical Interface Issues

Physical layer (Layer 1) issues are the most common cause of network outages.

Identify issues in cables, connectors, and ports that can affect network performance:

⚠️ Incorrect cables:

Using multimode instead of single mode fiber, or mismatching CAT 5/6/7/8, can degrade performance.

📶 Signal degradation:

Caused by crosstalk, EMI, and attenuation. Use cable testers to detect.

🔌 Improper termination:

Mismatched pins or poor crimps can cause CRC errors or disconnections.

🔄 TX/RX mismatch:

Transmitter and receiver lines crossed will disrupt communication.

❌ Interface errors:

CRC errors, runts, giants, or high drop counts may signal damaged cables or connectors.

🔌 Port issues:

Admin-down or error-disabled status typically results from misconfiguration or hardware faults.

⚡ PoE issues:

Power budget exceeded or incorrect standards (802.3af vs 802.3at) may prevent devices from functioning.

5.3 Troubleshooting Network Services

Common service-related problems include:

• Switching: STP misconfigurations (loops, incorrect port roles/states), VLAN mismatches, and ACL blocks.
• Routing: Incorrect default routes, broken static/dynamic routes, and incomplete routing tables.
• Addressing: IP conflicts, incorrect subnet masks, gateway errors, and DHCP scope exhaustion.
• Wireless: Poor signal coverage, overlapping channels, roaming issues, and client disassociations.

5.4 Troubleshooting Performance Issues

Diagnose performance degradation due to:

• Congestion: Too many simultaneous users or lack of QoS policies.
• Contention: Competing devices trying to access the same medium.
• Bottlenecks: Limited bandwidth or underpowered routers/switches.
• Latency, jitter, packet loss: Especially noticeable in VoIP/video traffic. Use tools to monitor round-trip times and buffer delays.

5.5 Troubleshooting Tools and Commands

Use the right software and hardware tools to isolate and fix problems:

• CLI tools: ping, traceroute, ipconfig/ifconfig, netstat, nslookup, dig, arp, tcpdump.
• Discovery tools: nmap for port scanning; LLDP/CDP for device discovery.
• Speed/packet testers: Validate throughput and diagnose delays using speed testers and protocol analyzers.
• Physical tools: Toner probes, cable testers, Wi-Fi analyzers, taps, and visual fault locators.
• Device commands: show interface, show vlan, show mac-address-table, show route, show config, show arp, show power.