CCNA Network Infrastructure and Connectivity Practice Question
Which TWO interface issues can be identified by analyzing the output of the 'show interfaces' command?
⚠ Common exam trap
Cisco often tests that 'show interfaces' reveals physical-layer issues like duplex and speed mismatches, but candidates mistakenly think it also shows Layer 2 or Layer 3 problems such as VLAN or routing misconfigurations.
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
✓
Duplex mismatch
The 'show interfaces' command displays interface statistics and operational status, including duplex and speed settings. A duplex mismatch occurs when one end of a link is set to full-duplex and the other to half-duplex, leading to collisions and CRC errors visible in the output. Speed mismatch is also detectable because the interface will show the negotiated speed or errors like 'input errors' if the speeds do not match.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Duplex mismatch
Why this is correct
A duplex mismatch occurs when one device is manually set to full duplex and the other is auto-negotiating or set to half duplex, often on a point-to-point link. The full-duplex side does not defer and sends anytime, while the half-duplex side only transmits when it senses the wire free, leading to collisions and late collisions on the half-duplex side; 'show interfaces' will display late collisions, runts, CRC errors, or excessive input errors on one or both sides. This is a classic Layer 1/physical problem that shows up clearly in interface counters.
- ✗
Routing protocol misconfiguration
Why it's wrong here
A misconfigured routing protocol (e.g., wrong OSPF area, incorrect EIGRP AS number, or bad neighbor statements) prevents routes from being exchanged, but the underlying physical interfaces remain up and operational. 'show interfaces' would show clean counters and a working link, so it cannot reveal that routes are missing from the routing table; you would use 'show ip protocols' to verify the configuration and 'show ip route' or 'show ip ospf neighbor' to see if adjacencies are established.
- ✓
Speed mismatch
Why this is correct
If two devices are configured with different speeds (e.g., 100 Mbps vs 1 Gbps) and auto-negotiation is disabled or fails, the link will either fail to come up or come up with the wrong speed, resulting in a down/down or up/down state in 'show interfaces'. Even if the interface shows up, the speed mismatch prevents meaningful communication because the physical layer cannot properly exchange frames, leading to excessive alignment errors or no connectivity; the 'show interfaces' output will show the speed and duplex settings, making it apparent.
- ✗
VLAN mismatch
Why it's wrong here
A VLAN mismatch occurs when two switches on opposite ends of a trunk have different native VLANs or allowed VLAN lists, so frames are placed into incorrect VLANs or dropped. The interface itself remains up/up and the physical layer is fine, so 'show interfaces' will not reveal any CRC, collisions, or line protocol errors; instead, you must compare 'show interfaces trunk' or 'show vlan' on both devices to see the inconsistency.
- ✗
STP topology change
Why it's wrong here
Spanning Tree Protocol topology changes occur when a port transitions to forwarding or blocking, often due to a link flap or a new switch joining, and they cause MAC address table flushing, but they do not alter the counters or status shown in 'show interfaces' output. The interface may still show up/up with no errors, so the command reveals nothing about STP events; you'd need 'show spanning-tree' to see port roles/states or 'debug spanning-tree events' to watch topology change notifications.
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.
✓Duplex mismatchCorrect answer▾
Why this is correct
A duplex mismatch occurs when one device is manually set to full duplex and the other is auto-negotiating or set to half duplex, often on a point-to-point link. The full-duplex side does not defer and sends anytime, while the half-duplex side only transmits when it senses the wire free, leading to collisions and late collisions on the half-duplex side; 'show interfaces' will display late collisions, runts, CRC errors, or excessive input errors on one or both sides. This is a classic Layer 1/physical problem that shows up clearly in interface counters.
✗Routing protocol misconfigurationWrong answer — click to see why▾
Why this is wrong here
Routing protocol misconfiguration is a Layer 3 issue that does not manifest in the 'show interfaces' output, which focuses on Layer 1 and Layer 2 interface statistics. To diagnose routing issues, you would use commands like 'show ip route' or 'show ip protocols'.
Why candidates choose this
Students might think that interface errors could be caused by routing problems, but routing protocols operate above the interface level and do not affect interface counters directly.
✗VLAN mismatchWrong answer — click to see why▾
Why this is wrong here
A VLAN mismatch is a Layer 2 issue that does not appear in the 'show interfaces' output; it is diagnosed using 'show vlan' or 'show interfaces trunk'. The 'show interfaces' command shows physical and data link layer statistics, not VLAN membership.
Why candidates choose this
Students may confuse interface errors with VLAN mismatches because both involve Layer 2, but VLAN mismatches are not reflected in interface counters like CRC errors or collisions.
✗STP topology changeWrong answer — click to see why▾
Why this is wrong here
STP topology changes are not directly visible in 'show interfaces'; they are monitored using 'show spanning-tree' or 'debug spanning-tree events'. The 'show interfaces' command does not provide information about spanning-tree state changes.
Why candidates choose this
Students might think that interface counters could indicate STP issues, but STP topology changes are control plane events that do not affect interface statistics like input errors or 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?”
Go deeper
Related to this question
Learn chapter
Half-Duplex vs Full-Duplex
Key term
CRC
CRC (Cyclic Redundancy Check) is an error-detecting code used to detect accidental changes to raw data in digital networks.
Key term
Duplex mismatch
A duplex mismatch occurs when two connected network devices are configured with different duplex settings (one half-duplex, one full-duplex), leading to poor performance and data errors.
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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.