- A
Duplex mismatch: the interface with late collisions is half-duplex while the other end is full-duplex. Configure both ends to auto-negotiate speed and duplex.
Late collisions only occur on half-duplex Ethernet when multiple stations attempt to transmit simultaneously. When one end is full-duplex, the half-duplex end perceives any overlapping transmission as a collision, producing late collisions and CRC errors. Auto-negotiation correctly sets both ends to full-duplex when supported, fixing the problem.
- B
Speed mismatch: one interface is set to 100 Mbps, the other to 1 Gbps. This causes frequent link flaps, resulting in CRC errors. Use the 'speed' command to match the rates.
Why wrong: Speed mismatches typically prevent link establishment entirely; the interface would show 'down/down' or 'up/down', not stable up with late collisions. Late collisions are specific to half-duplex operation, not speed differences.
- C
Faulty Ethernet cable causing signal degradation, which leads to CRC errors. The late collisions are a result of the switch misdetecting collisions due to the degraded signal. Replace the cable.
Why wrong: A faulty cable can cause CRC errors, but it will not generate late collisions unless the problem causes autonegotiation to fall back to half-duplex — which is a duplex mismatch, not a cable fault per se. Late collisions are a protocol-layer symptom, not physical-layer.
- D
A broadcast storm caused by a loop is flooding the uplink with frames, leading to CRC errors and late collisions as the switch discards excess traffic. Enable Spanning Tree Protocol to block the redundant path.
Why wrong: Broadcast storms generate massive amounts of traffic but do not directly cause CRC errors or late collisions. Late collisions are a layer-2 contention mechanism in half-duplex Ethernet, unrelated to loop-induced flooding. CRC errors from a storm would require physical bit errors, which aren’t inherent to an Ethernet frame storm.
Quick Answer
The answer is a duplex mismatch, where the interface with late collisions is operating in half-duplex while the other end is in full-duplex. This occurs because the half-duplex interface uses CSMA/CD and detects collisions after the transmission window—late collisions—when the full-duplex side transmits without waiting, corrupting frames and causing CRC errors on both ends. On the CCNA 200-301 v2 exam, this scenario tests your understanding of how duplex mismatch manifests differently on each side: one interface shows late collisions (half-duplex), while the other shows only CRC errors (full-duplex). A common trap is assuming both interfaces should show the same error types; remember that late collisions are exclusive to half-duplex links. For a memory tip, think “Late means Half”—if you see late collisions on one side, that interface is half-duplex, and the mismatch is confirmed.
CCNA Network Infrastructure and Connectivity Practice Question
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. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. 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 network administrator is troubleshooting slow file transfers between two servers in different access-layer switches. The administrator runs the 'show interface' command on the uplink connecting the two switches and notices a high number of CRC errors on both ends, but a high number of late collisions on only one interface; the other interface reports no late collisions.
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 interface with late collisions is half-duplex while the other end is full-duplex. Configure both ends to auto-negotiate speed and duplex.
A is correct because a duplex mismatch causes one interface to operate in half-duplex (detecting late collisions due to CSMA/CD) while the other operates in full-duplex (no collisions). The half-duplex interface waits for the carrier sense before transmitting, but the full-duplex interface transmits immediately, causing the half-duplex side to detect collisions after the transmission window (late collisions). CRC errors occur on both ends because frames are corrupted when collisions happen. Configuring both ends to auto-negotiate ensures matching duplex and speed, resolving the issue.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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: the interface with late collisions is half-duplex while the other end is full-duplex. Configure both ends to auto-negotiate speed and duplex.
Why this is correct
Late collisions only occur on half-duplex Ethernet when multiple stations attempt to transmit simultaneously. When one end is full-duplex, the half-duplex end perceives any overlapping transmission as a collision, producing late collisions and CRC errors. Auto-negotiation correctly sets both ends to full-duplex when supported, fixing the problem.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Speed mismatch: one interface is set to 100 Mbps, the other to 1 Gbps. This causes frequent link flaps, resulting in CRC errors. Use the 'speed' command to match the rates.
Why it's wrong here
Speed mismatches typically prevent link establishment entirely; the interface would show 'down/down' or 'up/down', not stable up with late collisions. Late collisions are specific to half-duplex operation, not speed differences.
- ✗
Faulty Ethernet cable causing signal degradation, which leads to CRC errors. The late collisions are a result of the switch misdetecting collisions due to the degraded signal. Replace the cable.
Why it's wrong here
A faulty cable can cause CRC errors, but it will not generate late collisions unless the problem causes autonegotiation to fall back to half-duplex — which is a duplex mismatch, not a cable fault per se. Late collisions are a protocol-layer symptom, not physical-layer.
- ✗
A broadcast storm caused by a loop is flooding the uplink with frames, leading to CRC errors and late collisions as the switch discards excess traffic. Enable Spanning Tree Protocol to block the redundant path.
Why it's wrong here
Broadcast storms generate massive amounts of traffic but do not directly cause CRC errors or late collisions. Late collisions are a layer-2 contention mechanism in half-duplex Ethernet, unrelated to loop-induced flooding. CRC errors from a storm would require physical bit errors, which aren’t inherent to an Ethernet frame storm.
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 mismatch: the interface with late collisions is half-duplex while the other end is full-duplex. Configure both ends to auto-negotiate speed and duplex.Correct answer▾
Why this is correct
Late collisions only occur on half-duplex Ethernet when multiple stations attempt to transmit simultaneously. When one end is full-duplex, the half-duplex end perceives any overlapping transmission as a collision, producing late collisions and CRC errors. Auto-negotiation correctly sets both ends to full-duplex when supported, fixing the problem.
✗Speed mismatch: one interface is set to 100 Mbps, the other to 1 Gbps. This causes frequent link flaps, resulting in CRC errors. Use the 'speed' command to match the rates.Wrong answer — click to see why▾
Why this is wrong here
Confuses speed mismatch (which prevents link establishment) with duplex mismatch (which allows the link to come up but causes errors and late collisions).
✗Faulty Ethernet cable causing signal degradation, which leads to CRC errors. The late collisions are a result of the switch misdetecting collisions due to the degraded signal. Replace the cable.Wrong answer — click to see why▾
Why this is wrong here
Assumes all interface errors are cable-related and overlooks the characteristic late-collision signature of duplex mismatch.
✗A broadcast storm caused by a loop is flooding the uplink with frames, leading to CRC errors and late collisions as the switch discards excess traffic. Enable Spanning Tree Protocol to block the redundant path.Wrong answer — click to see why▾
Why this is wrong here
Misinterprets high utilization as a source of physical-layer errors and ignores the diagnostic value of asymmetric late 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
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between CRC errors (which can have multiple causes like cable faults or duplex mismatch) and late collisions (which are a definitive indicator of a duplex mismatch), leading candidates to incorrectly attribute CRC errors alone to a cable issue.
Trap categories for this question
Command / output trap
Speed mismatches typically prevent link establishment entirely; the interface would show 'down/down' or 'up/down', not stable up with late collisions. Late collisions are specific to half-duplex operation, not speed differences.
Detailed technical explanation
How to think about this question
Late collisions occur when a frame is transmitted and a collision happens after the first 512 bits (the slot time for 10/100 Mbps Ethernet) have been sent, indicating the cable segment is too long or, more commonly, a duplex mismatch. In a duplex mismatch, the half-duplex side uses CSMA/CD and waits for the carrier sense, but the full-duplex side transmits without checking, causing the half-duplex side to detect a collision after it has already started sending. CRC errors on both ends result from the corrupted frames that are partially transmitted during these late collisions.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Questions learners often ask
What does this 200-301 question test?
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Duplex mismatch: the interface with late collisions is half-duplex while the other end is full-duplex. Configure both ends to auto-negotiate speed and duplex. — A is correct because a duplex mismatch causes one interface to operate in half-duplex (detecting late collisions due to CSMA/CD) while the other operates in full-duplex (no collisions). The half-duplex interface waits for the carrier sense before transmitting, but the full-duplex interface transmits immediately, causing the half-duplex side to detect collisions after the transmission window (late collisions). CRC errors occur on both ends because frames are corrupted when collisions happen. Configuring both ends to auto-negotiate ensures matching duplex and speed, resolving the issue.
What should I do if I get this 200-301 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
4 more ways this is tested on 200-301
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network administrator is troubleshooting connectivity between two directly connected Cisco switches. Hosts on VLAN 10 connected to Switch A cannot ping the default gateway on Switch B. The interface on Switch A shows 'up/up' but there are excessive CRC errors and runts. The administrator checks the interface configuration on both switches. What is the most likely cause of the issue?
hard- A.Replace the faulty Ethernet cable.
- ✓ B.Mismatched duplex and speed settings between the interfaces.
- C.Disable spanning tree on both interfaces to prevent loop prevention from blocking traffic.
- D.Assign the interfaces to the same VLAN to ensure Layer 2 connectivity.
Why B: The presence of excessive CRC errors and runts on an interface that is 'up/up' strongly indicates a Layer 1 or Layer 2 duplex mismatch. When one switch is set to full-duplex and the other to half-duplex (or auto-negotiation fails), the half-duplex side will detect collisions and retransmit, while the full-duplex side will not, leading to frame corruption (CRC errors) and truncated frames (runts). Configuring both interfaces with the same duplex and speed settings (e.g., full-duplex and 1000 Mbps) resolves this mismatch, restoring proper connectivity for VLAN 10 traffic to the default gateway.
Variation 2. You are connected to R1. The network administrator reports that the link between R1 and R2 is flapping and performance is poor. Examine the provided show interface output on R1, identify the root cause of the issue, and apply the necessary configuration fix to resolve the problem permanently.
hard- ✓ A.Configure the interface with the 'duplex full' command.
- B.Configure the interface with the 'speed 100' command.
- C.Configure the interface with the 'no shutdown' command.
- D.Configure the interface with the 'duplex auto' command.
Why A: The show interface output on R1 indicates the interface is operating in half-duplex with high CRC errors and input errors, classic symptoms of a duplex mismatch when the remote side (R2) is set to full-duplex. The root cause is that R1’s duplex setting does not match R2’s, causing collisions and flapping. Configuring 'duplex full' on R1’s GigabitEthernet0/1 forces full-duplex, which resolves the mismatch if the remote side is already forced full. Option B (speed 100) only configures speed—it does not change duplex, so the mismatch persists. Option C (no shutdown) is irrelevant because the interface is administratively up (the issue is operational). Option D (duplex auto) would set R1 to autonegotiate, but if R2 is forced full, autonegotiation fails and defaults to half-duplex, recreating the mismatch. Therefore, only 'duplex full' permanently fixes the issue.
Variation 3. A network administrator is troubleshooting connectivity issues between two switches. Hosts connected to Switch A cannot ping hosts on Switch B. The link between the switches shows as up/up on both ends, but interface error counters (CRC, runts) are increasing rapidly. What should the administrator do to resolve the issue?
hard- A.Replace the faulty cable between the two switches.
- ✓ B.Change the duplex setting on SwitchA's interface to auto-negotiation.
- C.Replace the SFP module on SwitchA's GigabitEthernet0/1 port.
- D.Enable the interface with 'no shutdown' command.
Why B: The increasing CRC and runts errors on an up/up link indicate a duplex mismatch, where one side is using full duplex and the other half duplex. Assuming SwitchB is using auto-negotiation (the default on Cisco switches), changing SwitchA's interface to auto-negotiation will allow both ends to negotiate the correct duplex setting, resolving the mismatch and stopping the errors.
Variation 4. You are connected to R1. The network administrator reports that hosts on VLAN 10 cannot communicate with the server attached to R2's GigabitEthernet0/1 interface. Troubleshoot and resolve the issue. Identify the root cause and apply the necessary fix on R1.
hard- ✓ A.Configure 'no duplex' and 'no speed' under interface GigabitEthernet0/0 to enable auto-negotiation.
- B.Change the duplex setting on GigabitEthernet0/0 to half-duplex using 'duplex half'.
- C.Apply 'speed 100' and 'duplex full' on GigabitEthernet0/0 to match a common switch configuration.
- D.Clear the interface counters on GigabitEthernet0/0 with 'clear counters gigabitethernet0/0' without changing any configuration.
Why A: The issue is a duplex mismatch on GigabitEthernet0/0. R1 is configured with 'duplex full' and 'speed 1000', but the connected switch port is likely set to auto-negotiate or is set to half-duplex. This causes high input errors (1500) and degraded performance. The fix is to set R1's G0/0 to auto-negotiate both speed and duplex, matching the switch's configuration. Enter interface configuration mode for G0/0, issue 'no duplex' and 'no speed' to revert to auto, then clear the interface counters with 'clear counters gigabitethernet0/0'.
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Last reviewed: Jun 25, 2026
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