CCNA Network Infrastructure and Connectivity Practice Question
A switch port and a host NIC have a duplex mismatch. Which symptom is most likely?
⚠ Common exam trap
Don't confuse duplex mismatch symptoms with total connectivity loss or latency-only issues; focus on error and collision symptoms.
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
✓
Increased late collisions and poor performance
A duplex mismatch often causes collisions, frame errors, and degraded throughput, especially on the half-duplex side. It is a classic physical/link layer performance problem.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Increased late collisions and poor performance
Why this is correct
A duplex mismatch occurs when one side of an Ethernet link runs full-duplex while the other runs half-duplex. The half-duplex side fails to sense the full-duplex side's transmission, so both transmit simultaneously, producing late collisions after the 64-byte collision window. These late collisions corrupt frames, forcing retransmissions that degrade throughput, increase latency, and cause poor performance.
- ✗
Incorrect VLAN tagging on trunks
Why it's wrong here
VLAN tagging on trunks uses 802.1Q tags to identify VLANs, which is independent of link duplex settings. A duplex mismatch affects the physical signaling and collision behavior on the wire, not the contents of the Ethernet frame's tag field. Therefore, incorrect VLAN tagging is a configuration error on the switch, not a symptom of a duplex mismatch.
When this WOULD be correct
In a scenario where a question asks about VLAN tagging issues on a trunk link, specifically mentioning misconfigured VLANs or mismatched VLAN IDs between switches, option B would be the correct answer. This could involve a situation where traffic is being incorrectly tagged due to VLAN misconfigurations.
- ✗
OSPF area mismatch errors
Why it's wrong here
OSPF area mismatch errors are a Layer 3 routing protocol issue, unrelated to physical or data-link layer duplex negotiation. Duplex mismatch may drop packets, but OSPF adjacency still forms if IP connectivity exists; an area mismatch prevents neighbor formation because Hello packets carry area IDs. Thus, a duplex mismatch does not generate OSPF-specific area errors.
When this WOULD be correct
If the question were about OSPF configuration issues, such as identifying the cause of routing problems in a network with multiple OSPF areas, then an option mentioning OSPF area mismatch errors would be correct. For example, a question could ask what symptoms indicate a misconfigured OSPF area in a multi-area setup.
- ✗
A change in the subnet mask on the host
Why it's wrong here
Changing the subnet mask on a host alters the IP addressing and routing logic at Layer 3, but duplex mismatch is a Layer 1 physical and Layer 2 data-link negotiation problem. The subnet mask determines the network portion of an IP address, while duplex mode affects how frames are sent and received on the media. These are separate protocol layers, so a duplex mismatch would never alter IP addressing parameters.
When this WOULD be correct
In a question about network configuration issues, if it asks about the effects of modifying the subnet mask on a host's ability to communicate with other devices, option D would be correct if it leads to communication failures due to incorrect subnetting.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Increased late collisions and poor performanceCorrect answer▾
Why this is correct
A duplex mismatch occurs when one side of an Ethernet link runs full-duplex while the other runs half-duplex. The half-duplex side fails to sense the full-duplex side's transmission, so both transmit simultaneously, producing late collisions after the 64-byte collision window. These late collisions corrupt frames, forcing retransmissions that degrade throughput, increase latency, and cause poor performance.
✗Incorrect VLAN tagging on trunksWrong answer — click to see why▾
Why this is wrong here
VLAN tagging on trunks is a Layer 2 function that deals with VLAN identification using 802.1Q tags. Duplex mismatch is a physical-layer issue affecting how data is sent and received (simultaneous vs. one direction at a time) and has no impact on VLAN tagging.
★ When this WOULD be the correct answer
In a scenario where a question asks about VLAN tagging issues on a trunk link, specifically mentioning misconfigured VLANs or mismatched VLAN IDs between switches, option B would be the correct answer. This could involve a situation where traffic is being incorrectly tagged due to VLAN misconfigurations.
Why candidates choose this
Students might confuse 'mismatch' in general, thinking any mismatch (duplex or VLAN) causes similar symptoms. However, VLAN tagging errors lead to connectivity issues within specific VLANs, not collisions.
✗OSPF area mismatch errorsWrong answer — click to see why▾
Why this is wrong here
OSPF area mismatch errors are Layer 3 routing protocol issues that prevent OSPF neighbors from forming. Duplex mismatch is a Layer 1/2 problem that affects frame delivery and collision detection, not routing protocol adjacency.
★ When this WOULD be the correct answer
If the question were about OSPF configuration issues, such as identifying the cause of routing problems in a network with multiple OSPF areas, then an option mentioning OSPF area mismatch errors would be correct. For example, a question could ask what symptoms indicate a misconfigured OSPF area in a multi-area setup.
Why candidates choose this
Both involve mismatches, and students might think any mismatch causes performance problems. However, OSPF area mismatches result in routing table incompleteness, not collisions.
✗A change in the subnet mask on the hostWrong answer — click to see why▾
Why this is wrong here
A change in subnet mask is a Layer 3 IP configuration change that affects network/host identification. Duplex mismatch is a physical-layer issue and does not alter IP addressing or subnet masks.
★ When this WOULD be the correct answer
In a question about network configuration issues, if it asks about the effects of modifying the subnet mask on a host's ability to communicate with other devices, option D would be correct if it leads to communication failures due to incorrect subnetting.
Why candidates choose this
Students might think that any misconfiguration (duplex or subnet) leads to poor performance. However, subnet mask changes cause reachability issues, not collisions.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
Half-Duplex vs Full-Duplex
Key term
Frame
A frame is a unit of data at the Data Link layer of networking, containing header, payload, and trailer information for local network delivery.
Key term
NIC
A NIC is a hardware component that connects a computer to a network, enabling communication over wired or wireless media.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 200-301 exam.