Network + Section 5

Network troubleshooting methodology

A structured, repeatable process for identifying, diagnosing, and resolving network problems.

Identify the problem

The first step of troubleshooting: gather information, question users, and identify symptoms and recent changes.

Establish a theory of probable cause

A troubleshooting step involving questioning the obvious and considering multiple possible explanations for the issue.

Test the theory

A troubleshooting step where the proposed cause is verified before proceeding to implement a solution.

Establish a plan of action

A troubleshooting step involving creating a plan to resolve the problem and identifying potential effects of the solution.

Implement the solution

A troubleshooting step involving applying the fix, escalating if necessary.

Verify full system functionality

A troubleshooting step confirming the issue is resolved and implementing preventive measures if applicable.

Document findings

The final troubleshooting step, recording the problem, cause, and solution for future reference.

Fiber mismatch

A cabling issue caused by connecting incompatible fiber types (e.g., single-mode to multi-mode) or wavelength mismatches.

Cable category

A standardized rating (e.g., Cat 5e, Cat 6, Cat 6a) indicating a copper cable's supported speed and bandwidth.

Crosstalk

Interference caused by signals from one wire pair bleeding into an adjacent pair within a cable.

EMI (Electromagnetic Interference)

Interference from external electrical or magnetic sources that can degrade signal quality on a cable.

Attenuation

The gradual loss of signal strength as it travels over a distance through a cable.

Improper termination

A cabling issue caused by incorrectly wired or crimped connectors, leading to intermittent or failed connections.

TX/RX reversed

A cabling issue where transmit and receive wires are swapped, preventing proper communication.

CRC error (Cyclic Redundancy Check)

An error indicating a frame was corrupted during transmission and failed an integrity check.

Runt

An Ethernet frame that is shorter than the minimum allowed size (64 bytes), often caused by collisions.

Giant

An Ethernet frame that exceeds the maximum allowed size, often due to misconfiguration.

Drop (interface)

A packet that is discarded by an interface, often due to congestion or errors.

Error disabled state

A switch port state where the interface is automatically shut down due to a detected error condition (e.g., port security violation).

PoE (Power over Ethernet)

A technology that delivers electrical power to devices over standard Ethernet cabling.

PoE standards (802.3af/at/bt)

IEEE standards defining different power delivery levels for Power over Ethernet.

Transceiver mismatch

A hardware issue caused by connecting incompatible transceiver types or speeds between devices.

dB loss (signal strength)

A measurement of signal power loss, used to calculate whether a fiber or transceiver link will function properly.

Switching loop

A condition where duplicate frames circulate endlessly on a network due to redundant Layer 2 paths without loop prevention.

VLAN assignment issue

A misconfiguration where a device is placed in the wrong VLAN, causing connectivity or segmentation problems.

ACL misconfiguration

An error in access control list rules that unintentionally blocks or allows traffic.

Routing table issue

A problem where a router's table contains incorrect, missing, or conflicting route entries.

Address pool exhaustion

A condition where a DHCP server runs out of available IP addresses to assign to new clients.

Duplicate IP address

A conflict that occurs when two devices on the same network are assigned the same IP address.

Congestion (network)

A condition where network demand exceeds available capacity, resulting in delays or packet loss.

Bottleneck

A point in the network where capacity is limited, constraining overall performance.

Latency

The delay between sending and receiving data across a network.

Packet loss

The failure of one or more transmitted packets to reach their destination.

Jitter

Variation in the delay of received packets, which can disrupt real-time applications like VoIP and video.

Wireless interference

Disruption of a wireless signal caused by competing devices, physical obstructions, or overlapping frequencies.

Channel utilization

A measure of how much a wireless channel's available capacity is being used, impacting performance when high.

Roaming misconfiguration

An issue where a wireless client fails to properly transition between access points, causing dropped or degraded connections.

Protocol analyzer

A tool (e.g., Wireshark) used to capture and inspect network packets for troubleshooting and analysis.

Port scanner

A tool used to identify open ports and running services on a network device.

Device discovery (tool)

A tool or process used to automatically identify devices present on a network.

Speed test

A tool used to measure the throughput and performance of a network connection.

ping

A command-line tool that tests reachability of a host by sending ICMP echo requests and measuring response.

traceroute (tracert)

A command-line tool that displays the path packets take to a destination, showing each hop along the way.

nslookup

A command-line tool used to query DNS servers and resolve domain names to IP addresses (or vice versa).

dig

A command-line DNS lookup tool that provides more detailed query output than nslookup.

tcpdump

A command-line packet capture tool used to inspect network traffic in real time.

netstat

A command-line tool that displays active network connections and listening ports on a device.

Tone generator and probe

A hardware tool used to trace and identify individual cables within a bundle or wall.

Cable tester

A hardware tool used to verify that a cable is wired correctly and free of faults.

Tap (network tap)

A hardware device that copies network traffic passively for monitoring or analysis without disrupting the flow.

Port mirror (SPAN)

A switch feature that copies traffic from one or more ports to another port for monitoring purposes.

Wi-Fi analyzer

A tool used to assess wireless signal strength, channel usage, and interference in a wireless environment.

show route

A common network device command used to display the current routing table.

show interface

A common network device command used to display the status and statistics of a specific interface.

show config

A common network device command used to display the current configuration of a device.