Quick Answer
The answer is distance, link speed, and frame size. These three factors are critical because buffer credits compensate for the propagation delay inherent in long-haul FC links; distance determines the round-trip time, link speed dictates how many bits are in flight per second, and frame size affects how many frames are needed to fill the link pipe. On the Cisco DCCOR 350-601 exam, this concept tests your understanding of Fibre Channel buffer-to-buffer credit flow control, often appearing in a drag-and-drop or multiple-select scenario where a common trap is to overlook frame size or to confuse buffer credits with BB_credits for ISL oversubscription. A reliable memory tip is the “D-S-F” mnemonic: Distance, Speed, Frame—each directly scales the credit count, so if you double the distance or speed, you double the credits, while smaller frames require more credits to maintain throughput.
This 350-601 practice question tests your understanding of storage network. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which THREE factors should be considered when sizing buffer credits for a long-haul FC link?
Link speed
Higher speed requires more credits to keep the link busy.
Number of targets
Why wrong: Target count affects zoning, not credit requirements on a single link.
Frame size
Larger frames reduce overhead but increase credit delay; used to calculate credits.
Distance
Longer distance requires more credits to avoid stalls.
Buffer-to-buffer credit pool size
Why wrong: This is what you configure, not a factor to consider.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Link speed
Link speed (A) is a critical factor because higher speeds require more buffer credits to maintain full throughput over a given distance. The buffer credit requirement scales linearly with link speed, as each credit represents the ability to send one frame before receiving an acknowledgment. For example, a 16 Gbps link needs twice as many buffer credits as an 8 Gbps link for the same distance.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Link speed
Why this is correct
Higher speed requires more credits to keep the link busy.
Related concept
Read the scenario before looking for a memorised answer.
Number of targets
Why it's wrong here
Target count affects zoning, not credit requirements on a single link.
Frame size
Why this is correct
Larger frames reduce overhead but increase credit delay; used to calculate credits.
Related concept
Read the scenario before looking for a memorised answer.
Distance
Why this is correct
Longer distance requires more credits to avoid stalls.
Related concept
Read the scenario before looking for a memorised answer.
Buffer-to-buffer credit pool size
Why it's wrong here
This is what you configure, not a factor to consider.
Common exam traps
Cisco often tests the misconception that the number of targets or initiators influences buffer credit requirements, when in fact only distance, speed, and frame size matter for the per-link calculation.
Detailed technical explanation
Buffer credits implement flow control in Fibre Channel using the BB_Credit mechanism defined in FC-FS-4. The formula is: BB_Credit = (Distance in km * Link Speed in Gbps * 1000) / (Frame Size in bytes * 8 / 10). The 8/10 accounts for 8b/10b encoding overhead. In real-world scenarios, long-haul links (e.g., 100 km at 32 Gbps) may require thousands of credits, often exceeding default switch port buffer pools, necessitating buffer credit expansion or dedicated long-haul optics.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 350-601 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
Storage Network — This question tests Storage Network — Read the scenario before looking for a memorised answer..
The correct answer is: Link speed — Link speed (A) is a critical factor because higher speeds require more buffer credits to maintain full throughput over a given distance. The buffer credit requirement scales linearly with link speed, as each credit represents the ability to send one frame before receiving an acknowledgment. For example, a 16 Gbps link needs twice as many buffer credits as an 8 Gbps link for the same distance.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Same concept, more angles
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. Which THREE factors should be considered when calculating the required number of buffer credits for a long-distance Fibre Channel link? (Choose three.)
hardWhy A: To calculate buffer credits, you need the distance, data rate, and frame size. Option A: distance affects propagation delay. Option C: data rate affects how many frames can be in flight. Option E: frame size (maximum) determines how many bytes per credit. Option B is wrong because VSAN count does not matter. Option D is wrong because number of zones is irrelevant.
Last reviewed: Jun 24, 2026
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