Section 5: Network Troubleshooting Flashcards
What is the first step in the troubleshooting process?
Identify the Problem
After identifying the problem, what is the next step in troubleshooting?
Establish a theory of probable cause
After establishing a theory of probable cause, what is the next step?
Test the Theory
Once the theory is confirmed, what is the next step?
Establish a plan of action
After establishing a plan of action, what is the next step?
Implement the solution
After implementing the solution, what is the next step?
Verify full system functionality, if applicable install preventive measures.
After verifying full system functionality, what is the next step?
Document findings and actions
What are common cable issues, and how can incorrect cables cause problems?
- Single Mode vs. Multimode Fiber: Using the wrong type can cause mismatched connections and poor performance.
- Category 5/6/7/8: Lower-category cables may not support higher bandwidths or frequencies.
- STP vs. UTP: Incorrect use can result in inadequate shielding and increased interference.
What are 3 common causes of signal degradation in cables?
Crosstalk: Signals interfere between adjacent cables, often caused by poor cable design or installation.
Interference: External electromagnetic interference (EMI) from devices or power lines.
Attenuation: Signal weakening over long distances or through poor-quality cables.
What is improper termination, and how does it affect networks?
Terminating cables incorrectly (e.g., poor crimping, incorrect pinouts) can cause data loss or connection instability.
What happens when TX/RX are transposed, and how can you fix it?
Issue: Transmitting and receiving wires are reversed, preventing communication.
Fix: Verify and correct the cable pinout or use a crossover cable if required.
What are interface issues indicated by increasing interface counters?
Cyclic Redundancy Check (CRC): Indicates data corruption during transmission.
Runts: Packets smaller than the minimum allowed size.
Giants: Packets exceeding the maximum allowed size.
Drops: Packets discarded due to buffer overflow or errors.
What are 3 common port status issues, and what do they mean?
Error Disabled: Port shut down due to detected issues (e.g., security violation).
Administratively Down: Port manually disabled by an administrator.
Suspended: Port inactive due to configuration or policy conflicts.
What are common hardware issues related to Power over Ethernet (PoE)?
Power Budget Exceeded: Insufficient power available for connected PoE devices.
Incorrect Standard: Device uses a PoE standard incompatible with the switch (e.g., IEEE 802.3af vs. 802.3at).
What are common transceiver issues, and how can they affect performance?
Mismatch: Incorrect transceiver type (e.g., single-mode used on a multimode connection).
Signal Strength Issues: Weak or poor-quality signal due to dirty connectors, long distances, or degraded optics.
What is the purpose of STP (Spanning Tree Protocol)?
Prevents network loops by creating a loop-free topology in Ethernet networks.
* Ensures redundancy while avoiding broadcast storms.
What are 4 common STP issues?
- Network Loops: Occur when STP is disabled or misconfigured, leading to broadcast storms.
- Root Bridge Selection: If the wrong switch becomes the root bridge, it can cause suboptimal paths.
- Port Roles: Incorrect roles (root, designated, or blocked) can affect traffic flow.
- Port States: Misconfigured port states (e.g., listening, learning, forwarding, or blocking) can delay convergence or cause outages.
What happens with an incorrect VLAN assignment, and how do you resolve it?
Issue: Devices assigned to the wrong VLAN cannot communicate with the intended network.
Resolution: Verify and correct VLAN configurations on switch ports and trunk links.
What issues can occur with Access Control Lists (ACLs)?
Incorrect ACL Rules: Can block legitimate traffic or allow unauthorized access.
Resolution: Review ACLs for proper source, destination, and protocol/port configurations.
What is a routing table, and why is it critical for route selection?
Routing Table: Contains information about network destinations and the best paths to reach them.
Issues:
* Missing or incorrect routes can prevent traffic from reaching its destination.
* Incorrect default routes can send traffic to the wrong next hop.
What happens with an incorrect default gateway configuration?
Devices cannot communicate outside their local subnet.
- Resolution: Configure the correct gateway IP address on affected devices.
What is address pool exhaustion, and what causes it?
Definition: No available IP addresses for new devices.
Causes: Mismanaged DHCP scopes or too many devices on the network.
Resolution: Increase the DHCP scope or implement subnetting.
What are the impacts of an incorrect IP address or duplicate IP address?
Incorrect IP Address: Prevents devices from communicating on the network.
Duplicate IP Address: Causes conflicts, leading to intermittent or no connectivity for affected devices.
Resolution: Verify and assign unique, correct IP addresses to all devices.
What is congestion/contention, and how does it affect the network?
Congestion: Occurs when network traffic exceeds available resources, causing delays.
Contention: Multiple devices competing for the same resources.
Impact: Increased latency, packet loss, and reduced throughput.
What is bottlenecking, and how can it be resolved?
Bottlenecking: A single device or link limits overall network performance.
* Fix: Upgrade the bottlenecked hardware (e.g., switches, routers) or optimize traffic flow.
How do bandwidth and throughput capacity differ?
Bandwidth: Maximum theoretical data rate of a network connection.
Throughput Capacity: Actual amount of data successfully transmitted over the network.
Issue: Low throughput indicates inefficiencies or congestion.
Fix: Identify and resolve congestion, upgrade hardware, or implement Quality of Service (QoS).
What is latency, and how can it be reduced?
Latency: The delay in data transmission between source and destination.
* Fix: Reduce hop counts, upgrade equipment, or optimize routing paths.
What is packet loss, and what are common causes?
Packet Loss: Packets are dropped or fail to reach their destination.
Causes: Congestion, faulty cables, interference, or overloaded devices.
Fix: Identify and replace faulty hardware or optimize traffic flow.
What is jitter, and why is it problematic?
Jitter: Variability in packet delay, often impacting real-time applications like VoIP.
* Fix: Implement QoS to prioritize real-time traffic and reduce variability.
What are common wireless interference issues, and how can they be mitigated?
Channel Overlap: Adjacent access points using overlapping channels.
* Fix: Use non-overlapping channels (e.g., 1, 6, 11 on 2.4 GHz).
Other sources: Nearby electronics, walls, or microwave ovens.
* Fix: Relocate devices or switch to the 5 GHz or 6 GHz band.
What causes signal degradation or loss, and how is it resolved?
Causes: Distance from the AP, physical obstructions, or interference.
Fix: Optimize AP placement, upgrade antennas, or use repeaters/extenders.
What is insufficient wireless coverage, and how can it be fixed?
Issue: Dead zones where wireless signals cannot reach.
Fix: Add additional access points, mesh networks, or extenders to improve coverage.
What are client disassociation issues, and how can they be addressed?
Issue: Devices disconnect frequently from the network.
Fix: Check for AP capacity limits, interference, or misconfigurations in power settings.
What is roaming misconfiguration, and how does it affect users?
Issue: Devices fail to transition smoothly between access points.
Fix: Enable features like fast roaming (802.11r) or optimize AP power levels and channel selection.
What does a protocol analyzer do?
It captures and analyzes network traffic to identify issues, monitor performance, or detect malicious activity. Example: Wireshark.
What is Nmap used for?
Scans networks for open ports, services, and vulnerabilities.
What is Link Layer Discovery Protocol (LLDP) or Cisco Discovery Protocol (CDP)?
Protocols used for device discovery on local networks, providing details like device type and IP address.
What is a speed tester?
Measures the upload and download speeds of a network connection, often used to diagnose bandwidth issues.
What does the ping command do?
Sends ICMP Echo Request packets to test connectivity to a remote host and measure round-trip time.
What does the traceroute/tracert command do?
Displays the path packets take to reach a destination and identifies latency at each hop.
What is the purpose of the nslookup command?
Queries DNS servers to resolve domain names into IP addresses or troubleshoot DNS issues.
What is the purpose of the dig command?
Queries DNS servers to retrieve detailed information, such as A, MX, or TXT records.
What does tcpdump do?
Captures and displays network packets in real time, often used for troubleshooting network issues.
What does the netstat command show?
Displays network connections, routing tables, and protocol statistics.
What is the function of the ip/ifconfig/ipconfig commands?
They display or configure IP address information on network interfaces.
ipconfig: Windows.
ifconfig/ip: Linux/Unix.
What does the arp command do?
Displays or modifies the ARP table, which maps IP addresses to MAC addresses.
What is a toner used for in networking?
Locates specific wires or cables within a bundle or wall using a signal tone.
What does a cable tester do?
Tests the integrity of cables, checking for continuity, shorts, or incorrect wiring.
What are taps in networking?
Hardware devices that passively copy network traffic for monitoring or analysis.
What is the function of a Wi-Fi analyzer?
Scans wireless networks to identify signal strength, channel usage, and interference.
What is a visual fault locator?
Uses visible light to identify breaks or faults in optical fiber cables.
What does the show mac-address-table command do?
Displays the MAC address table of a switch, mapping MAC addresses to specific ports.
What does the show route command display?
Displays the routing table, showing how packets will be forwarded through the network.
What does the show config command display?
Shows the current configuration of a networking device.
What is the purpose of the show interface command?
Displays detailed information about the status, speed, and errors of network interfaces.
What does the show arp command do?
Displays the ARP table, showing IP-to-MAC address mappings.
What does the show vlan command display?
Lists VLANs configured on a switch and their associated ports.
What does the show power command do?
Displays power-related information, often for devices using Power over Ethernet (PoE).
