This 200-301 practice question tests your understanding of network infrastructure and connectivity. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: a duplex mismatch occurs when one device operates in full-duplex mode and the other in half-duplex mode on the same link, causing communication errors.. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A switch port and a host NIC have a duplex mismatch. Which symptom is most likely?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Increased late collisions and poor performance
Correct. Duplex mismatch commonly leads to collisions and poor performance.
Incorrect VLAN tagging on trunks
Why wrong: VLAN tagging is unrelated to duplex mismatch.
OSPF area mismatch errors
Why wrong: OSPF area mismatches are Layer 3 control-plane issues, not duplex symptoms.
A change in the subnet mask on the host
Why wrong: Duplex does not alter IP addressing parameters.
Answer choices
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.
Key principle: A duplex mismatch occurs when one device operates in full-duplex mode and the other in half-duplex mode on the same link, causing communication errors.
Answer analysis
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
Correct. Duplex mismatch commonly leads to collisions and poor performance.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
A duplex mismatch occurs when one device operates in full-duplex mode and the other in half-duplex mode on the same link, causing communication errors.
Incorrect VLAN tagging on trunks
Why it's wrong here
VLAN tagging is unrelated to 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 mismatches are Layer 3 control-plane issues, not duplex symptoms.
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
Duplex does not 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
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.
Why this is correct
Correct. Duplex mismatch commonly leads to collisions and poor performance.
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.
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.
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?”
Common exam traps
Don't confuse duplex mismatch symptoms with total connectivity loss or latency-only issues; focus on error and collision symptoms.
Detailed technical explanation
Duplex mismatch occurs when one device on a link operates in full-duplex mode while the other operates in half-duplex mode. Full-duplex allows simultaneous sending and receiving of data without collisions, whereas half-duplex requires devices to take turns transmitting, which can cause collisions if not properly synchronized. This mismatch leads to performance degradation because the half-duplex device detects collisions that the full-duplex device does not expect, resulting in late collisions and retransmissions. In Cisco networking environments, duplex settings can be configured manually or negotiated automatically via auto-negotiation protocols. When auto-negotiation fails or is disabled on one side, a duplex mismatch can occur. The half-duplex side will experience increased late collisions and frame errors, causing poor throughput and network performance. Troubleshooting involves verifying and aligning duplex settings on both the switch port and the host NIC to ensure consistent operation. The exam trap lies in confusing duplex mismatch symptoms with Layer 3 or VLAN-related issues. For example, OSPF area mismatches or VLAN tagging problems do not cause collisions or duplex-related errors. Practically, duplex mismatches manifest as physical/link layer issues affecting frame transmission reliability, so recognizing collision-related symptoms is key to diagnosing duplex mismatches in Cisco networks.
Key takeaway
A duplex mismatch occurs when one device operates in full-duplex mode and the other in half-duplex mode on the same link, causing communication errors.
Real-world example
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Review a duplex mismatch occurs when one device operates in full-duplex mode and the other in half-duplex mode on the same link, causing communication errors., then practise related 200-301 questions on the same topic to reinforce the concept.
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FAQ
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — A duplex mismatch occurs when one device operates in full-duplex mode and the other in half-duplex mode on the same link, causing communication errors..
The correct answer is: 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.
Review a duplex mismatch occurs when one device operates in full-duplex mode and the other in half-duplex mode on the same link, causing communication errors., then practise related 200-301 questions on the same topic to reinforce the concept.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
A duplex mismatch occurs when one device operates in full-duplex mode and the other in half-duplex mode on the same link, causing communication errors.
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Last reviewed: May 17, 2026
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