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A+ Core 1 · CompTIA 220-1201 V15 · Objective 5.7

Given a scenario, troubleshoot network issues

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Objective 5.7: Given a scenario, troubleshoot network issues

Cert: CompTIA A+ Core 1 (220-1201) V15 Domain: 5.0 Hardware and Network Troubleshooting Weight: ~28% of Core 1 (largest domain) Depth: Given a scenario, troubleshoot. Recognize network symptoms and apply the right diagnostic steps.

What this objective tests

You should recognize common network problems (intermittent wireless, slow speeds, jitter, VoIP quality, latency, authentication failures, port flapping, intermittent internet) and know how to isolate and resolve them.

Key facts

Common symptoms:

  • Intermittent wireless connectivity. Wi-Fi drops, reconnects, drops again. Causes: weak signal, channel congestion, AP/router issues, client NIC driver bugs, interference from microwaves, neighboring APs.
  • Slow network speeds. Real throughput below expectation. Could be ISP throughput, wired duplex mismatch, Wi-Fi channel congestion, an overloaded switch port, malware uploading data, or a saturated NAT table.
  • Limited connectivity. Windows phrase meaning the device has an IP but cannot reach the broader network or internet. Often a gateway, DNS, or DHCP issue.
  • Jitter. Variation in packet arrival timing. Bad for real-time apps (VoIP, video, gaming). Caused by congestion, queueing delays, or unstable wireless.
  • Poor VoIP quality. Symptoms: choppy audio, dropped words, latency. Causes: jitter, packet loss, insufficient bandwidth, QoS not configured, busy network during peak hours.
  • Port flapping. A switch port goes up and down repeatedly. Causes: bad cable, failing NIC, duplex mismatch, spanning tree issues. Often shows in switch logs.
  • High latency. Long round-trip times. Causes: distance (satellite, far-away servers), overloaded ISP equipment, faulty WAN circuit, routing problems.
  • External interference. RF sources (microwaves, cordless phones, baby monitors) degrade 2.4 GHz Wi-Fi. Walls and metal absorb signal.
  • Authentication failures. Wrong password, expired credentials, RADIUS server issues, certificate problems on 802.1X enterprise Wi-Fi.
  • Intermittent internet connectivity. ISP-side outages, modem/ONT issues, router/firewall instability, DNS problems.

Diagnostic approach

  1. Start with the affected user. Is it just them? Same problem at another desk?
  2. If just them: check their device's IP config (ipconfig /all on Windows, ifconfig or ip addr on Linux/macOS).
  3. If multiple users: check the switch/AP/firewall. Look for port flapping or alerts.
  4. Ping by IP, then by name. Distinguishes layer 3 from DNS issues.
  5. Traceroute to see where latency or losses happen.
  6. Check the ISP status page or call them for suspected outages.
  7. For Wi-Fi: use a Wi-Fi analyzer to check signal strength and channel.

Common gotchas

  • Wi-Fi vs Ethernet. Always test both if possible. A laptop on Wi-Fi may show poor performance that a wired connection would not.
  • APIPA address. 169.254.x.x means DHCP failed. Internet won't work.
  • VPN affecting speeds. A user complaining of slow internet may be routing all traffic through a corporate VPN with limited bandwidth.
  • DNS slow or wrong. Web pages take a long time to load or load to the wrong site. Try ping 8.8.8.8 to confirm internet, then ping google.com to test DNS.
  • Spanning Tree loops. Mis-cabled network creates a loop, which spanning tree usually catches but can cause port flapping or broadcast storms during convergence.
  • Authentication failures vs DHCP failures. Both can produce "can't connect to Wi-Fi" complaints. Authentication failure usually means wrong password or cert. DHCP failure produces APIPA.
  • 2.4 GHz interference. Common in apartment buildings. Move to 5 GHz if the device supports it.

Real-world context

Common helpdesk patterns:

  • "Internet is slow." Run a speed test wired and on Wi-Fi. Compare to plan. Test from multiple devices.
  • "Wi-Fi keeps disconnecting." Wi-Fi analyzer first. Then driver update on the client. Then AP placement.
  • "My VoIP calls sound choppy." Check upstream bandwidth, jitter, packet loss. Configure QoS to prioritize voice. Move the phone to wired if possible.
  • "I can ping but can't reach websites." DNS issue. Try a different DNS server (1.1.1.1, 8.8.8.8).
  • "Several users in the same area lost internet at the same time." Switch port, AP, or upstream uplink issue.

Sources

  • [CompTIA A+ 220-1201 Exam Objectives Version 4.0, Section 5.5](../../../../../../30-RevyTechJourney-Build/../30-RevyTechJourney/CompTIA%20A%2B%20220-1201%20Exam%20Objectives%20%284.0%29.pdf)
  • [Wikipedia: Network performance](https://en.wikipedia.org/wiki/Network_performance)
  • [Wikipedia: Jitter](https://en.wikipedia.org/wiki/Jitter)
  • [Wikipedia: Quality of service](https://en.wikipedia.org/wiki/Quality_of_service)
  • [Microsoft Learn: Troubleshoot network connection issues](https://learn.microsoft.com/en-us/windows/client-management/troubleshoot-tcpip)