CRC Errors on 16 Gbps FC Links: Cable Distance Impact
An engineer is troubleshooting a SAN performance issue. The MDS switch shows high CRC errors on an F port. The host is connected via a 16 Gbps FC link. The errors increase when the host sends large I/O. What is the most likely cause?
Quick Answer
The correct answer is that the cable length exceeds the supported distance for 16 Gbps Fibre Channel. At 16 Gbps, signal integrity degrades sharply over long distances, causing bit errors that the MDS switch detects as CRC errors on the F port. This degradation becomes more pronounced with large I/O because longer frames have a higher probability of containing corrupted bits, making the error count spike under heavy workloads. On the Cisco DCCOR and CCNP Data Center Core 350-601 exam, this scenario tests your understanding of physical-layer limitations in high-speed SAN designs, often appearing as a trap where candidates blame transceivers or SFP failures instead of distance. Remember that for 16 Gbps FC, OM3 multimode fiber supports only about 10 meters, while OM4 extends to roughly 100 meters—exceeding these limits directly causes CRC errors. A simple memory tip: “16 Gbps, 16 meters? No—check the OM3/OM4 spec before blaming the SFP.”
⚠ Common exam trap
Cisco often tests the distinction between physical-layer errors (CRC, running disparity) and higher-layer issues (buffer credits, zoning), so the trap here is that candidates confuse buffer credit starvation (a flow-control problem) with CRC errors (a signal-integrity problem).
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
✓
The cable length exceeds the supported distance for 16 Gbps.
At 16 Gbps Fibre Channel, signal integrity degrades over long cable distances, causing bit errors that manifest as CRC errors on the F port. The error increase with large I/O is characteristic of marginal signal quality, as larger frames are more likely to encounter corrupted bits. Option E correctly identifies that the cable length exceeds the supported distance for 16 Gbps, typically 10 meters for OM3 multimode fiber or 100 meters for OM4, depending on the transceiver type.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The host's HBA firmware is outdated.
Why it's wrong here
Firmware issues can cause problems but typically not exclusively CRC errors on the switch side.
- ✗
The zone configuration is incorrect.
Why it's wrong here
Incorrect zoning prevents communication but does not cause CRC errors.
- ✗
The host is experiencing buffer credit starvation.
Why it's wrong here
Buffer credit starvation results in discards (credit loss), not CRC errors.
- ✗
The switch port is configured for 8 Gbps.
Why it's wrong here
Speed mismatch would cause link issues but not necessarily CRC errors.
- ✓
The cable length exceeds the supported distance for 16 Gbps.
Why this is correct
16 Gbps FC has shorter reach; long cables cause CRC errors, especially under heavy load.
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Same concept, more angles
1 more way this is tested on 350-601
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. Refer to the exhibit. The interface shows CRC errors. What is the most likely cause of these errors?
medium- A.Insufficient buffer credits for the distance.
- B.The port is configured as F port but should be E port.
- C.Speed mismatch between the ports.
- ✓ D.Faulty SFP or fiber cable.
Why D: CRC errors indicate that frames are being received with invalid checksums, which is typically caused by physical-layer issues such as a faulty SFP module or damaged fiber cable. These errors occur when the cyclic redundancy check computed by the receiver does not match the CRC appended by the sender, pointing to corruption during transmission. In Fibre Channel storage networks, a faulty SFP or cable is the most common root cause of CRC errors, as they directly affect signal integrity.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-601 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 350-601 exam.