Network+ N10-009 5.0 Network Troubleshooting

Identify the problem(Gather information)

Collect relevant data from network devices and logs

Identify the problem(Question users)

Ask users about issues and recent changes

Identify the problem(Identify symptoms)

Determine the observable signs of the problem

Identify the problem(Determine if anything has changed)

Check for recent configuration, hardware, or environmental changes

Identify the problem(Duplicate the problem, if possible)

Recreate the issue to understand it better

Identify the problem(Approach multiple problems individually)

Handle separate issues one at a time

Establish a theory of probable cause(Question the obvious)

Consider simple explanations first

Establish a theory of probable cause(Consider multiple approaches)

Use top-to-bottom or bottom-to-top OSI model and divide-and-conquer methods
Top-to-bottom/bottom-to-top OSI model - Check layers sequentially
Divide and conquer - Isolate and test components

Test the theory to determine the cause(If theory is confirmed, determine next steps to resolve problem)

Plan corrective actions based on findings

Test the theory to determine the cause(If theory is not confirmed, establish a new theory or escalate)

Re-evaluate hypothesis or involve higher-level support

Establish a plan of action to resolve the problem and identify potential effects

Develop step-by-step resolution plan and anticipate impacts

Implement the solution or escalate as necessary

Carry out plan or request assistance if needed

Verify full system functionality and implement preventive measures if applicable

Confirm systems operate correctly and prevent recurrence

Document findings, actions, outcomes, and lessons learned throughout the process

Record detailed troubleshooting process for reference

Cable issues(Incorrect cable)

Wrong cable type or configuration
Single mode vs. multimode - Fiber type difference
Category 5/6/7/8 - Cable performance rating
Shielded twisted pair (STP) vs. unshielded twisted pair (UTP) - Shielding differences

Cable issues(Signal degradation)

Loss or distortion of signals
Crosstalk - Interference between wires
Interference - External signal disruption
Attenuation - Signal weakening over distance

Cable issues(Improper termination)

Incorrect connector or pinout

Cable issues(Transmitter (TX)/Receiver (RX) transposed)

Cables connected in reverse

Interface issues(Increasing interface counters)

Rising error counts on interfaces
Cyclic redundancy check (CRC) - Data corruption detection
Runts - Undersized frames
Giants - Oversized frames
Drops - Lost packets

Interface issues(Port status)

Port administrative or operational state
Error disabled - Port shut due to errors
Administratively down - Disabled by configuration
Suspended - Temporarily inactive

Hardware issues(Power over Ethernet (PoE))

Supplying power via Ethernet
Power budget exceeded - Too many devices drawing power
Incorrect standard - Incompatible PoE type

Hardware issues(Transceivers)

Fiber or copper modules
Mismatch - Incorrect type or speed
Signal strength - Weak transmission

Switching issues(STP)

Spanning Tree Protocol for loop prevention
Network loops - Redundant path causing loops
Root bridge selection - Determines main switch
Port roles - Defines switch port function
Port states - Current operational state

Switching issues(Incorrect VLAN assignment)

Device in wrong VLAN

Switching issues(ACLs)

Access control lists restricting traffic

Route selection(Routing table)

Table of known routes

Route selection(Default routes)

Fallback route for unknown destinations

Address pool exhaustion

No available IP addresses

Incorrect default gateway

Wrong router for outbound traffic

Incorrect IP address(Duplicate IP address)

Same IP assigned to multiple devices

Incorrect subnet mask

Network division misconfiguration

Congestion/contention

Too much traffic on network

Bottlenecking

Traffic slowed by limited resource

Bandwidth(Throughput capacity)

Maximum data transfer rate

Latency

Delay in packet delivery

Packet loss

Dropped packets in transit

Jitter

Variation in packet arrival times

Wireless(Interference)

Signal disruption by other devices
Channel overlap - Frequencies interfering

Wireless(Signal degradation or loss)

Weak or lost wireless signal

Wireless(Insufficient wireless coverage)

Areas with no signal

Wireless(Client disassociation issues)

Clients repeatedly disconnect

Wireless(Roaming misconfiguration)

Incorrect handoff between APs

Software tools(Protocol analyzer)

Captures and analyzes traffic

Software tools(Command line)

Network commands for testing
ping - Check host reachability
traceroute/tracert - Trace network path
nslookup - Query DNS records
tcpdump - Capture packets
dig - DNS lookup tool
netstat - Active connections
ip/ifconfig/ipconfig - Interface info
arp - MAC to IP mapping

Software tools(Nmap)

Network scanning tool

Software tools(Link Layer Discovery Protocol (LLDP)/Cisco Discovery Protocol (CDP))

Device discovery protocols

Software tools(Speed tester)

Measure network throughput

Hardware tools(Toner)

Locate cables with signal tone

Hardware tools(Cable tester)

Check cable integrity

Hardware tools(Taps)

Passive network monitoring

Hardware tools(Wi-Fi analyzer)

Analyze wireless signals

Hardware tools(Visual fault locator)

Fiber troubleshooting light source

Basic networking device commands(show mac-address-table)

Display learned MAC addresses

Basic networking device commands(show route)

Display routing table

Basic networking device commands(show interface)

Interface status and errors

Basic networking device commands(show config)

Current device configuration

Basic networking device commands(show arp)

ARP table lookup

Basic networking device commands(show vlan)

Display VLAN info

Basic networking device commands(show power)

Show device power status