Network Troubleshooting Guide
Step-by-step methodology for systematic network troubleshooting
Systematic Troubleshooting Process
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1. Define the Problem
Clearly identify what is not working and gather information about the symptoms.
Key Questions & Actions:
- • What exactly is the problem?
- • When did it start occurring?
- • Is it affecting all users or specific ones?
- • What error messages are displayed?
- • Can you reproduce the problem consistently?
2
2. Gather Information
Collect relevant data including error messages, network topology, and recent changes.
Key Questions & Actions:
- • Review system and network logs
- • Check recent configuration changes
- • Document network topology
- • Identify affected systems and users
- • Note the scope and impact of the issue
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3. Consider Possibilities
List potential causes based on the OSI model layers and network components.
Key Questions & Actions:
- • Physical layer: cables, connectors, power
- • Data link layer: switches, VLANs, spanning tree
- • Network layer: routing, IP addressing
- • Transport layer: TCP/UDP, port issues
- • Application layer: services, protocols
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4. Create Action Plan
Develop a systematic approach to test each possibility, starting with the most likely causes.
Key Questions & Actions:
- • Prioritize based on probability and impact
- • Start with simple, non-disruptive tests
- • Plan for rollback procedures
- • Consider maintenance windows for changes
- • Prepare necessary tools and documentation
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5. Implement & Test
Execute your plan methodically, testing one change at a time.
Key Questions & Actions:
- • Make one change at a time
- • Test thoroughly after each change
- • Document what you tried
- • Monitor for side effects
- • Be prepared to rollback if needed
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6. Document Results
Record what worked, what didn't, and lessons learned for future reference.
Key Questions & Actions:
- • Document the root cause
- • Record the solution that worked
- • Update network documentation
- • Share knowledge with the team
- • Create preventive measures
Troubleshooting Tools by OSI Layer
Physical Layer (Layer 1)
Cable Tester
Test cable continuity and wiring
Multimeter
Check power and electrical connections
Optical Power Meter
Test fiber optic signal strength
Network Interface LEDs
Check link status and activity
Data Link Layer (Layer 2)
Switch Port Status
Check port configuration and status
MAC Address Table
Verify MAC learning and forwarding
VLAN Configuration
Check VLAN membership and trunking
Spanning Tree Status
Verify STP topology and port states
Network Layer (Layer 3)
ping
Test basic IP connectivity
traceroute
Trace packet path through network
Routing Table
Verify routing information
ARP Table
Check IP to MAC address resolution
Transport/Application Layers
telnet
Test TCP port connectivity
nslookup/dig
Test DNS resolution
netstat
Show network connections and listening ports
Wireshark
Capture and analyze network packets
Essential Troubleshooting Commands
Connectivity Testing
ping [destination]traceroute [destination]telnet [host] [port]nslookup [domain]System Information
show ip routeshow interfacesshow arpnetstat -rn