Capítulo 5 de 524% del examen
Network Troubleshooting
Troubleshooting restores service methodically rather than by guesswork. This chapter covers the structured troubleshooting methodology, essential command-line and hardware tools, and common cabling, wireless, and connectivity problems. A disciplined approach isolates faults quickly and prevents recurrence.
The Troubleshooting Methodology
A structured methodology guides technicians from symptom to solution consistently. Each step builds on the last, from identifying the problem to documenting the outcome. Following it prevents wasted effort and missed causes.
Identify the problem first
Gather information, question users, and determine the scope before acting.
Theorize, then test
Form a theory of probable cause and test it before implementing a fix.
Verify and document
Confirm full functionality, add preventive measures, and record the outcome.
Command-Line Diagnostic Tools
Software utilities reveal configuration and connectivity status at each layer. Knowing which tool answers which question speeds isolation. These commands are a technician's first line of investigation.
Test reachability with ping
ICMP echo requests confirm Layer 3 connectivity and round-trip time.
Map paths with traceroute
Hop-by-hop output locates where connectivity breaks or slows.
Check configuration and names
ipconfig shows local settings while nslookup diagnoses DNS resolution.
Inspect connections with netstat
View active sessions and listening ports to confirm services are running.
Hardware and Cabling Tools
Physical layer problems require hardware tools to diagnose and locate. Cable testers, tone generators, and optical meters confirm media integrity. Many outages trace back to simple cabling faults.
Verify pinouts with a cable tester
Confirm continuity and correct wiring, detecting opens, shorts, and miswires.
Locate cables with a toner and probe
A tone generator and inductive probe identify a specific wire in a bundle.
Inspect and clean fiber
Dirty or damaged connectors cause high loss and intermittent links.
Common Connectivity Issues
Many problems present recognizable symptoms that point to a specific cause. APIPA addresses, duplicate IPs, and duplex mismatches each have telltale signs. Matching symptom to cause accelerates resolution.
Recognize APIPA as DHCP failure
A 169.254.x.x address means the client could not reach a DHCP server.
Correlate CRC errors with physical faults
Rising CRC errors indicate damaged cabling or interference corrupting frames.
Spot duplex mismatches
Late collisions and poor throughput on a link suggest mismatched duplex settings.
Layer 2 and Wireless Faults
Switching loops and wireless coverage issues cause distinctive disruptions. Loops without STP flood the network, while weak signals cause disconnects. Understanding these helps resolve broad or intermittent outages.
Suspect loops behind broadcast storms
A redundant link without STP creates a Layer 2 loop that saturates the network.
Diagnose weak wireless signal
Attenuation and low signal-to-noise ratio cause disconnects far from the AP.
Watch for port security lockouts
An err-disabled port often reflects a port security violation to investigate.
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