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12 Network Trubleshooting - Coggle Diagram
12 Network Trubleshooting
1.0 Introduction to Network Troubleshooting
1.1 Importance: Minimizes downtime, restores service, reduces costs.
1.2 Goals:
Determine the problem's scope.
Identify the root cause.
Implement a solution.
Document the findings.
1.3 Troubleshooting Approach:
Systematic vs. Guessing
Based on models (e.g., OSI, TCP/IP).
2.0 Network Documentation
2.1 Why Document?
Baseline for "normal" operation.
Speeds up future troubleshooting.
Aids in network planning.
2.2 Key Documents:
Network Topology Diagrams: Physical and logical layouts.
Configuration Files: Router, switch, and server configs.
Addressing Schemes: IP addressing tables.
Baseline Performance Metrics: Normal traffic levels, utilization.
3.0 Troubleshooting Process (Structured Method)
3.1 Step 1: Identify the Problem
Gather information from users.
Define symptoms and scope.
Question: Who, What, When, Where?
3.2 Step 2: Establish a Theory of Probable Cause
Question the obvious.
Create a list of possible causes.
3.3 Step 3: Test the Theory to Determine the Cause
If theory confirmed: Proceed to next step.
If theory rejected: Go back to Step 2.
3.4 Step 4: Establish a Plan of Action & Implement the Solution
Plan the fix (e.g., change config, replace cable).
Implement the solution carefully.
3.5 Step 5: Verify Full System Functionality
Test the solution.
Confirm the problem is resolved.
Implement monitoring.
3.6 Step 6: Document the Findings, Actions, and Outcomes
Update documentation.
Record what was done and why.
4.0 Troubleshooting Tools
4.1 Software Tools:
Protocol Analyzers / Packet Sniffers: Wireshark.
Network Management Systems: Cisco Prime, SolarWinds.
Syslog Servers: Collect logs from network devices.
Cisco IOS Commands:
ping & traceroute (Connectivity)
show interfaces (Interface status)
show ip route (Routing table)
show vlan (VLAN information)
show running-config (Current configuration)
4.2 Hardware Tools:
Cable Testers: Verify physical cable integrity.
Multimeters: Measure voltage, current, resistance.
Network Analyzers: Advanced hardware for deep analysis.
5.0 Symptoms & Causes of Network Problems
5.1 Physical Layer Issues:
Symptoms: No connectivity, intermittent links, high error counts.
Causes: Damaged cables, faulty NICs, incorrect cabling (straight-through vs. crossover), duplex mismatches.
5.2 Data Link Layer Issues:
Symptoms: No connectivity on a VLAN, broadcast storms.
Causes: Incorrect VLAN assignment, missing/native VLAN mismatch, STP loops, port security violations.
5.3 Network Layer Issues:
Symptoms: Unable to reach remote networks, intermittent connectivity.
Causes: Incorrect IP address/subnet mask, wrong default gateway, routing protocol problems (e.g., OSPF neighbor adjacency down), ACLs blocking traffic.
5.4 Application Layer Issues:
Symptoms: A specific service (e.g., HTTP, FTP) is unavailable.
Causes: Service is down on the server, firewall rules, DNS resolution failure.
6.0 Troubleshooting IP Connectivity
6.1 The "Ping" Command Methodology:
Step 1: ping 127.0.0.1 (Loopback - Tests TCP/IP stack).
Step 2: ping local host IP address.
Step 3: ping default gateway.
Step 4: ping remote destination.
6.2 Common IP Connectivity Problems:
Routing Issues:
Missing or incorrect static/dynamic routes.
Check with show ip route.
DHCP Issues:
Client not receiving an IP address.
Check DHCP server scope, ip helper-address.
NAT Issues:
Inside local hosts cannot reach the internet.
Check NAT configuration and translations (show ip nat translations).
ACL Issues:
Legitimate traffic is blocked.
Check ACL rules and order (show access-lists).
