5.0 - Network Troubleshooting (copy)

Step 1: Identify The Problem

step in troubleshooting where you gather information, observe symptoms, question users, and determine what is not working, helping define the scope and nature of the issue

Step 2: Establish a Theory of Probable Cause

The step in troubleshooting where you develop possible explanations for the problem based on the gathered information, identifying the most likely cause of the issue

Step 3: Test the Theory to Determine the Cause

The troubleshooting step where you verify the suspected cause by testing the theory, confirming whether it actually explains the problem or if another cause must be investigated

Step 4: Establish a plan of action to resolve the problem and identify potential effects

The troubleshooting step where you create a strategy to fix the issue while considering possible impacts or risks the solution may have on systems, users, or the network

Step 5: Implement the solution or escalate as necessary

The troubleshooting step where you apply the planned fix to resolve the issue, or escalate the problem to higher-level support if it cannot be resolved at the current level

Step 6: Verify full system functionality and implement preventive measures if applicable

The troubleshooting step where you confirm the solution resolved the issue and that all systems are working properly, and apply measures to prevent the problem from happening again

Step 7: Document findings, actions, outcomes, and lessons learned throughout the process

The final troubleshooting step where you record the problem, the steps taken to resolve it, the results, and any lessons learned, helping improve future troubleshooting and maintain accurate records

Incorrect cable

A troubleshooting issue where the wrong type of cable is used for a connection, preventing devices from communicating properly or establishing a network link

Single Mode vs. Multimode

Incorrect Use: Using single mode fiber where

multimode is required, or vice versa, can cause signal

loss and inefficient data transmission.

Impact: This mismatch can result in increased

attenuation, poor signal quality, and reduced

bandwidth, affecting overall network performance

Category 5/6/7/8 Cable Issues

Incorrect Category: Using a lower category

cable (e.g., Cat5) instead of a higher

category (e.g., Cat6, Cat7, or Cat8) can

limit data transfer speeds and lead to

increased errors

Impact: This can cause network

slowdowns, increased latency, and an

inability to support high-speed

Shielded Twisted Pair (STP) vs. Unshielded Twisted Pair (UTP) Cable Issues

Incorrect Shielding: Using UTP

cables in environments with high

electromagnetic interference

(EMI) instead of STP can result in

signal degradation and data

corruption

Impact: This can lead to frequent

data retransmissions, increased

error rates, and reduced network

reliability and performance

Signal Degradation

A condition where a network signal weakens or loses quality as it travels over distance or through interference, which can lead to slow speeds, errors, or connection loss

Crosstalk

A form of signal interference where signals from one cable or wire pair interfere with another, causing data errors, reduced performance, or degraded communication quality

Interference

A disruption in a network signal caused by external sources such as electrical devices, radio frequencies, or other signals, which can degrade performance or cause data transmission errors

Attenuation

The gradual weakening of a signal as it travels through a transmission medium (such as copper or fiber cables), which can reduce signal quality and limit transmission distance

Improper Termination

A cabling issue where connectors or cable ends are not installed correctly, leading to poor signal transmission, connectivity problems, or network performance issues

Transmitter (TX)/Receiver (RX) Transposed

A cabling or connection issue where the transmit (TX) and receive (RX) wires or fibers are reversed, preventing devices from properly sending and receiving data

Interface Issues

Problems related to a network device’s port or interface, such as misconfiguration, hardware faults, or disabled interfaces, which can prevent proper communication between devices

Increasing Interface Counters

A troubleshooting indicator where error or packet counters on a network interface continue to rise, suggesting issues like collisions, dropped packets, or transmission errors affecting network performance

Cyclic Redundancy Check (CRC) Errors

Errors detected when a received data frame fails a CRC integrity check, indicating that the data was corrupted during transmission, often due to interference, cabling issues, or hardware problems

Runts

Ethernet frames that are smaller than the minimum valid frame size (less than 64 bytes), often caused by collisions, network congestion, or faulty network hardware

Giants

Ethernet frames that are larger than the maximum allowed frame size (typically greater than 1518 bytes without jumbo frames), often caused by misconfigurations, faulty network devices, or transmission errors

Drops

Packets that are discarded by a network device before reaching their destination, often due to network congestion, buffer overflow, errors, or resource limitations

Port status issues

Problems where a network port is down, administratively disabled, or not operating correctly, preventing devices from establishing proper connections or communication

Error Disabled

A network switch port state where the port is automatically shut down by the device due to a detected error or security violation, preventing network traffic until the issue is resolved and the port is re-enabled

Administratively Down

A network port status where the port has been manually disabled by a network administrator, preventing it from sending or receiving traffic until it is manually enabled

Suspended

A network port or interface status where the port is temporarily disabled or inactive, often due to policy enforcement, security issues, or administrative action, preventing normal data transmission until it is reactivated

Power over Ethernet (PoE) Issues

Problems where a network device fails to receive or supply power over an Ethernet cable, which can be caused by insufficient power budget, faulty cables, incompatible devices, or configuration errors, leading to device malfunctions or connectivity loss

Power Budget Exceeded

A Power over Ethernet (PoE) issue that occurs when a switch or power source cannot provide enough power for all connected PoE devices, causing some devices to not receive power or function properly

Incorrect Standard

A network or PoE issue that occurs when a device or cable does not support the required networking or PoE standard (e.g., using a non-PoE cable for a PoE device), leading to connectivity failures or insufficient power delivery

Transceiver Issues

Problems with a network transceiver (such as an SFP or SFP+) that prevent proper sending or receiving of signals, often caused by faulty hardware, incompatible modules, dirty connectors, or incorrect configuration

Mismatched Transceivers

A network issue that occurs when connected transceivers are incompatible (e.g., different speeds, types, or standards), resulting in link failures, poor performance, or no connectivity

Signal Strength

A measure of the power or quality of a transmitted signal in a network or wireless system; weak signal strength can cause slow speeds, dropped connections, or communication errors

Switching Issues

Problems within a network switch that affect the forwarding of traffic, such as misconfigurations, hardware failures, spanning-tree problems, or MAC table issues, which can lead to packet loss, loops, or connectivity disruptions

STP in Switching Issues

Spanning Tree Protocol (STP) problems occur when loops, misconfigurations, or topology changes in a switched network cause network instability, blocked ports, or disrupted traffic flow, impacting connectivity and performance

STP and Network Loops

Spanning Tree Protocol (STP) – A Layer 2 protocol that prevents network loops by blocking redundant paths in a switched network, ensuring only one active path exists at a time.

Network Loops – Occur when there are multiple active paths between switches, causing frames to circulate endlessly, leading to broadcast storms, MAC table instability, and network outages

Root Bridge Selection

The process in Spanning Tree Protocol (STP) where switches elect a single root bridge (central reference point) based on the lowest Bridge ID (priority + MAC address), which determines the optimal path and network topology

STP Port Roles

Defined roles assigned to switch ports in Spanning Tree Protocol (STP) to prevent loops and manage traffic flow:

• Root Port (RP) – The port with the best path to the root bridge on a non-root switch.

• Designated Port (DP) – The port responsible for forwarding traffic on a network segment.

• Blocked Port (Non-Designated) – A port placed in a blocking state to prevent loops and does not forward traffic

STP Port States

The different conditions a switch port goes through in Spanning Tree Protocol (STP) to ensure a loop-free network:

• Blocking – Does not forward frames; only listens for STP information to prevent loops.

• Listening – Prepares to participate in forwarding; processes STP frames but does not forward data yet.

• Learning – Begins to populate the MAC address table but still does not forward frames.

• Forwarding – Fully operational; forwards frames and learns MAC addresses.

• Disabled – Port is administratively shut down and does not participate in STP.

Incorrect VLAN Assignment

A network issue where a device or port is placed in the wrong VLAN, causing connectivity problems, inability to reach resources, or communication failures with intended network segments

Access Control Lists (ACLs)

Sets of rules used by network devices to permit or deny traffic, based on criteria such as IP addresses, protocols, and ports, helping to control access and enhance network security

Route Selection Issues

Problems that occur when a router chooses an incorrect or suboptimal path for forwarding traffic, often due to misconfigurations, incorrect metrics, routing protocol issues, or outdated routing tables, leading to inefficiency or connectivity problems

Routing Table Issues

Problems where a router’s routing table contains missing, incorrect, or outdated routes, causing traffic to be misrouted, dropped, or unable to reach its destination

Default Route Issues

Problems that occur when a default route (gateway of last resort) is missing, incorrect, or misconfigured, causing traffic to fail when no specific route exists for a destination network

Address Pool Exhaustion

A condition where a DHCP server runs out of available IP addresses to assign, preventing new devices from joining the network or obtaining a valid IP configuration

Incorrect Default Gateway

A network configuration issue where a device is assigned the wrong gateway address, preventing it from communicating with external networks or reaching destinations outside its local subnet

Incorrect IP Address

A configuration issue where a device is assigned an invalid or mismatched IP address, causing communication failures, network conflicts, or inability to reach other devices or networks

Duplicate IP Address

A network issue where two devices are assigned the same IP address, leading to IP conflicts, connectivity problems, and intermittent network disruptions

Incorrect Subnet Mask

A configuration issue where a device has the wrong subnet mask, causing it to misidentify local vs. remote networks, which can lead to communication failures or routing problems

Congestion/Contention

A network condition where multiple devices compete for limited bandwidth, causing delays, packet loss, and reduced performance due to overloaded network resources

Bottlenecking

A performance issue where a single component (such as a link, device, or interface) limits the overall data flow, causing slower network speeds and reduced efficiency for the entire system

Bandwidth

The maximum amount of data that can be transmitted over a network connection in a given time, typically measured in bits per second (bps)

Throughput Capacity

The actual amount of data successfully transmitted over a network in a given time, reflecting real-world performance after accounting for overhead, errors, and network conditions

Latency

The time it takes for data to travel from source to destination across a network, typically measured in milliseconds (ms), affecting responsiveness and performance

Packet Loss

A network issue where data packets fail to reach their destination, leading to retransmissions, reduced performance, and potential disruptions in communication

Jitter

The variation in packet arrival times on a network, which can cause voice, video, or real-time applications to experience delays, choppy playback, or degraded quality

Wireless Issues

Problems affecting a wireless network’s performance or connectivity, including signal interference, weak signal strength, incorrect configurations, security problems, or device incompatibilities, leading to slow speeds, dropped connections, or unreliable communication

Wireless Interference

Disruption in a wireless network caused by external signals, devices, or physical obstacles that weaken or distort the wireless signal, resulting in slow speeds, dropped connections, or poor network performance

Channel Overlap

A wireless networking issue that occurs when neighboring Wi-Fi networks use overlapping channels, causing interference, reduced throughput, and degraded wireless performance

Signal Degradation or Loss

A wireless issue where the strength or quality of the signal weakens or disappears due to distance, obstacles, interference, or faulty hardware, leading to slow connections or dropped connectivity

Insufficient Wireless Coverage

A wireless networking problem where the Wi-Fi signal does not reach all required areas, leaving dead zones or weak signal spots, which results in poor connectivity or no access for devices

Client Disassociation Issues

A wireless network problem where devices are unexpectedly disconnected from the access point, often caused by signal interference, weak coverage, authentication problems, or misconfigured wireless settings

Roaming Misconfiguration

A wireless network issue where a client device fails to seamlessly switch between access points, often due to incorrect wireless settings, overlapping channels, or inconsistent security configurations, resulting in dropped connections or poor performance

Software Tools

Applications or programs used in networking and IT to monitor, troubleshoot, configure, or analyze systems and networks, such as packet analyzers, network scanners, and diagnostic utilities

Protocol Analyzer/Packet Capture

A software tool that monitors and captures network traffic, allowing administrators to analyze packets, diagnose issues, and troubleshoot network performance or security problems

Ping (Windows/Linux)

A network utility that tests connectivity between devices by sending ICMP echo requests and measuring response time and packet loss, helping to diagnose reachability and network latency

Traceroute/tracert (Windows)

A network diagnostic tool that maps the path packets take from a source to a destination, showing each hop along the route and the time taken, useful for identifying routing issues or network delays

Nslookup (Windows)

A command-line tool used to query Domain Name System (DNS) servers to obtain IP addresses, domain names, and DNS record information, helping troubleshoot name resolution and DNS issues

Tcpdump (Linux)

A command-line network packet analyzer for capturing and inspecting network traffic in real-time, allowing administrators to diagnose network issues, monitor protocols, and troubleshoot connectivity problems

Dig (Linux)

A command-line tool used to query DNS servers for information about domain names, IP addresses, and DNS records, helping to troubleshoot DNS resolution and verify DNS configurations

netstat

A command-line tool that displays active network connections, listening ports, routing tables, and network statistics, useful for monitoring network activity and diagnosing connectivity or performance issues

ipconfig/ifconfig/ip

Displays IP configuration details such as IP address, subnet mask, and default gateway for network interfaces

Shows network interface settings and status, including IP address, MAC address, and link status

A more powerful replacement for ifconfig, used to view and configure network interfaces, routes, and addresses

arp

A protocol and command-line tool used to map IP addresses to their corresponding MAC addresses on a local network, helping devices locate each other and communicate within the same subnet

nmap

A network scanning tool used to discover devices, identify open ports, and detect services on a network, helping with security auditing, network inventory, and vulnerability assessment

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

A vendor-neutral protocol that advertises a device’s identity, capabilities, and neighbors on a local network

A Cisco proprietary protocol that shares similar information between Cisco devices

Speed Tester

A tool or application used to measure the performance of a network connection, including download and upload speeds, latency, and sometimes jitter, helping assess network quality and bandwidth availability

Toner

A network troubleshooting tool used with a toner probe kit to trace and identify cables in a network or telecommunication setup, helping locate cable endpoints or continuity issues

Cable Tester

A device used to verify the integrity and connectivity of network cables, checking for continuity, shorts, miswiring, or signal issues to ensure proper network performance

Taps

Hardware devices used to monitor network traffic without interrupting the flow, allowing administrators to capture, analyze, or troubleshoot data safely on live networks

Wi-Fi Analyzer

A tool or application used to scan and evaluate wireless networks, providing information on signal strength, channel usage, interference, and network performance to help optimize Wi-Fi coverage and troubleshoot connectivity issues

Visual Fault Locator

A fiber optic testing tool that uses a visible laser light to trace fiber cables, helping technicians identify breaks, bends, or faulty connections in fiber optic networks

show mac-address-table

A network switch command (commonly on Cisco devices) used to display the MAC address table, showing which MAC addresses are associated with specific switch ports, helping with troubleshooting, device identification, and verifying network connectivity

show route

A network device command used to display the routing table, showing known networks, associated next hops, and routing paths, helping administrators verify routes and troubleshoot connectivity issues

show interface

A network device command used to display the status and statistics of interfaces, including link status, errors, traffic, speed, duplex, and protocol information, helping to diagnose connectivity and performance issues

show config

A network device command used to display the current active configuration of a device, including interfaces, routing, VLANs, ACLs, and other settings, helping administrators verify configurations and troubleshoot issues

show arp

A network device command used to display the Address Resolution Protocol (ARP) table, showing IP-to-MAC address mappings for devices on the local network, useful for troubleshooting connectivity and identifying devices

show vlan

A network device command used to display VLAN information, including VLAN IDs, names, status, and associated ports, helping administrators verify VLAN configurations and troubleshoot network segmentation issues

show power

A network device command used to display the status of Power over Ethernet (PoE) on ports, including power availability, usage, and any faults, helping troubleshoot PoE-related issues and device power problems