Network Troubleshooting | CompTIA Network+ Study Guide

🧰 5.0 Network Troubleshooting

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

Identify the problem: Gather information, question users, identify symptoms, check for recent changes, and duplicate the issue if possible.
Establish a theory of probable cause: Question the obvious and consider multiple approaches (e.g., OSI top-to-bottom, divide and conquer).
Test the theory: If confirmed, move to the next step. If not, revise the theory or escalate.
Establish a plan of action: Develop a step-by-step strategy to resolve the issue and identify potential side effects.
Implement the solution: Apply the fix or escalate as needed.
Verify full functionality: Confirm that the issue is resolved and implement preventive measures if possible.
Document findings: Record what was done, including lessons learned, to aid future troubleshooting.

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.

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.

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.

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.