The correct first step to troubleshoot CRC errors on a trunk link is to check the physical cable and connectors for damage or loose connections. This is because CRC errors and runts are almost always symptoms of a Layer 1 physical-layer issue, such as faulty cabling, bent pins, or poor termination, which corrupts the frame’s cyclic redundancy check. On the ENCOR 350-401 exam, this question tests your ability to follow the OSI model hierarchy—always start at the physical layer before moving to configuration or protocol-level troubleshooting. A common trap is jumping to adjust duplex or speed settings, but those mismatches typically cause alignment errors or late collisions, not CRC errors. Remember the memory tip: “CRC means check the cable first.”
CCNP Network Assurance Practice Question
This 350-401 practice question tests your understanding of network assurance. 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 engineer is troubleshooting intermittent connectivity issues between two switches connected via a trunk link. The engineer notices that the port counters show a high number of CRC errors and runts on one side. Which action should the engineer take first?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the cable and connectors for damage or loose connections.
CRC errors and runts on a trunk link typically indicate a Layer 1 physical-layer issue, such as faulty cabling, damaged connectors, or poor termination. The first and most logical step is to inspect and test the physical cable and connectors, as this is the most common root cause and the easiest to verify before making configuration changes.
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.
✓
Check the cable and connectors for damage or loose connections.
Why this is correct
CRC errors and runts often indicate physical layer issues like faulty cabling.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Increase the MTU size on the interface.
Why it's wrong here
Increasing MTU can worsen fragmentation and does not fix physical errors.
✗
Configure the interface with a different duplex setting.
Why it's wrong here
Duplex mismatch causes collisions, not typically CRC errors.
✗
Disable Dynamic Trunking Protocol (DTP) on the interface.
Common exam trap: answer the scenario, not the keyword
Cisco often tests the principle that Layer 1 issues must be resolved first before considering Layer 2 or Layer 3 changes, and the trap here is that candidates jump to configuration changes (like duplex or DTP) instead of verifying the physical medium.
Detailed technical explanation
How to think about this question
CRC errors occur when the frame check sequence (FCS) computed by the receiving switch does not match the FCS appended by the sender, indicating bit corruption during transmission. Runts are frames smaller than 64 bytes (the minimum Ethernet frame size) and often result from collisions or transceiver issues on half-duplex links, but in modern full-duplex links they typically point to faulty hardware or cabling. A simple cable test with a time-domain reflectometer (TDR) or swapping the cable can quickly isolate the issue without risking service disruption.
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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Network Assurance — This question tests Network Assurance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Check the cable and connectors for damage or loose connections. — CRC errors and runts on a trunk link typically indicate a Layer 1 physical-layer issue, such as faulty cabling, damaged connectors, or poor termination. The first and most logical step is to inspect and test the physical cable and connectors, as this is the most common root cause and the easiest to verify before making configuration changes.
What should I do if I get this 350-401 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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