Troubleshoot Duplex Mismatch Causing Intermittent Network Drops
A technician is troubleshooting a workstation that loses network connectivity for a few seconds at random intervals. The technician observes that during the outage, the network icon in the system tray shows 'Unidentified network' or 'No internet access', but the IP configuration still shows a valid address from DHCP. The technician has already replaced the NIC and the Ethernet cable. Which of the following should the technician check NEXT?
⚠ Common exam trap
The trap here is that candidates often jump to DNS or firewall issues because of the 'No internet access' message, but the key clue is the 'Unidentified network' label and the fact that the IP address remains valid, which indicates a Layer 2 problem rather than a Layer 3 or above issue.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the speed and duplex settings on both the NIC and the switch port
The symptom of intermittent connectivity with a valid DHCP address but 'Unidentified network' or 'No internet access' points to a Layer 2 issue, often caused by a speed/duplex mismatch between the NIC and the switch port. When one side is set to auto-negotiation and the other is manually set (or fails to negotiate), the link may appear up but drops frames, causing brief outages. Since the NIC and cable have already been replaced, checking and aligning the speed and duplex settings on both ends is the logical next step.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Check the speed and duplex settings on both the NIC and the switch port
Why this is correct
A speed or duplex mismatch occurs when one side of a link is manually forced (e.g., 100 Mbps/full duplex) while the other is set to auto-negotiate; the auto side may fall back to half duplex, causing late collisions, CRC errors, and periodic packet loss even though the link light stays lit. This directly produces intermittent network drops rather than a complete loss of connectivity, making it the correct first thing to check.
- ✗
Verify that the DNS server is reachable and responsive
Why it's wrong here
DNS failures prevent name resolution, so a workstation might show 'No internet access' or fail to open websites, but the physical and data-link layers remain fully operational with a valid IP address. The network icon would typically indicate 'Connected, no internet' rather than a disconnected or 'Unidentified network' state, and connectivity would not drop intermittently. Because the symptom is a recurring loss of network traffic, a DNS server issue is an incorrect cause.
- ✗
Disable the Windows firewall temporarily
Why it's wrong here
A misconfigured Windows firewall can selectively block inbound or outbound application traffic, but it does not operate at the physical or data-link layer and cannot cause the adapter to drop its link or become an 'Unidentified network'. Intermittent total disconnection points to cabling, NIC, or switch-port issues, not packet-filtering rules, so disabling the firewall would not restore a stable link and could expose the system unnecessarily.
- ✗
Increase the DHCP lease time on the router
Why it's wrong here
DHCP lease expiration only causes a problem when the lease actually expires and renewal fails, at which point the client typically loses its assigned address and falls back to an Automatic Private IP Addressing (APIPA) address (169.254.x.x). That would be a complete loss of valid IP configuration, not brief intermittent outages while the workstation continues to hold a legitimate IP address. Shortening or extending lease time does not affect the negotiated link state or cause recurring disconnects.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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