CCNP Network Assurance Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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 are statistical counters that typically indicate frame corruption at the data-link layer, almost always rooted in physical-layer problems. Faulty patch cables, damaged RJ-45 connectors, improper termination, or electromagnetic interference near the cable can cause bit-level corruption that the NIC detects as cyclic redundancy check failures. Runts—frames shorter than 64 bytes—frequently accompany this condition when transceivers detect a signal loss or electrical anomaly mid-frame. Therefore, inspecting the physical path from the switchport through the patch panel to the end device is the first and most effective troubleshooting step.
- ✗
Increase the MTU size on the interface.
Why it's wrong here
Adjusting the MTU on an interface only changes the maximum IP packet size that the interface will transmit, which in turn affects IP fragmentation behavior, not the integrity of frames on the wire. CRC errors are generated when the receiving NIC recalculates the frame check sequence and finds a mismatch in bits that traversed the physical medium; no MTU change can recover bits lost to a bad cable or connector. In fact, increasing the MTU can make the problem worse, as larger frames are more susceptible to corruption and may lead to dropped packets if any intermediate device has a lower MTU. The condition is distinct from IP-level fragmentation issues, which would manifest as reassembly timeouts or incomplete packets rather than CRC errors.
- ✗
Configure the interface with a different duplex setting.
Why it's wrong here
Configuring a different duplex setting could actually be harmful, because a duplex mismatch—which occurs when one side runs full duplex and the other half duplex—causes late collisions and a high collision count, not an increase in CRC errors or runts. Modern Ethernet switches and NICs negotiate duplex automatically via auto-negotiation, and manually overriding this can introduce misconfiguration. Furthermore, CRC errors on a modern link are far more likely to be caused by degraded cabling or a faulty SFP, not by how the interface resolves its duplex state. Changing the duplex would not address the underlying frame-check-sequence failures and could exacerbate connectivity symptoms.
- ✗
Disable Dynamic Trunking Protocol (DTP) on the interface.
Why it's wrong here
Disabling Dynamic Trunking Protocol (DTP) is intended to stop an interface from attempting to negotiate a trunk link with a neighboring switch via periodic DTP advertisements; it has no effect on the physical transmission of data frames or the generation of FCS errors. CRC errors and runts originate from electrical conditions such as noise, signal attenuation, or a damaged cable that corrupts bits before they reach the receiving NIC. DTP operates at Layer 2 solely to decide whether an interface should become an access or trunk port, so it does not touch the mechanics of framing or bit integrity. Disabling DTP would only change how the switchport establishes its VLAN membership, not resolve any link-layer corruption.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 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 350-401 exam.