350-601 Storage Network Practice Question
A large enterprise data center has a disaster recovery site 100 km away. The SAN uses two MDS 9700 series switches at each site, connected via a dedicated dark fiber. Each link operates at 16 Gbps with a round-trip time of 2 ms. Recently, backup jobs to the remote storage array have been failing with timeout errors. The backup server is local to Site A, but the backup target is in Site B. The link utilization never exceeds 40%, and no errors are reported on the interfaces. The engineer suspects the issue is related to buffer credits. The current buffer credit count on the ISL is 16. The engineer calculates that for 16 Gbps over 100 km (2 ms RTT), they need at least 200 credits to maintain full throughput. Which action is most appropriate to resolve the issue?
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
✓
Increase the buffer-to-buffer credit count to 300 on the ISL interfaces.
The timeout errors during backup are caused by buffer credit starvation, as the current credit count of 16 is insufficient for the 100 km distance (2 ms RTT) at 16 Gbps. Increasing the buffer-to-buffer credit count to 300 ensures enough credits to maintain full throughput, preventing credit starvation and associated timeouts. Option B is correct because it directly addresses the insufficient credit count. Option A (NPIV) is unrelated to buffer credits; NPIV allows multiple N-port logins, not credit management. Option C (reducing speed to 8 Gbps) would lower throughput and is not optimal. Option D (FCIP) is unnecessary because the existing dark fiber provides a direct Fibre Channel link; FCIP is for IP-based transport and does not help with buffer credit issues.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Enable NPIV on the ISL ports to allow multiple logins.
Why it's wrong here
NPIV allows multiple N-port logins on a single port, but it does not address buffer credit starvation on long-distance ISLs.
- ✓
Increase the buffer-to-buffer credit count to 300 on the ISL interfaces.
Why this is correct
Increasing buffer-to-buffer credits to 300 provides enough credits to keep the link fully utilized over the 100 km distance, preventing timeout errors.
- ✗
Configure the ISL to operate at 8 Gbps to reduce the buffer credit requirement.
Why it's wrong here
Reducing the speed to 8 Gbps would reduce the required buffer credits but also halves the available bandwidth, which is not ideal for maintaining performance.
- ✗
Implement FCIP over the existing dark fiber to offload buffer credit management.
Why it's wrong here
FCIP encapsulates Fibre Channel over IP networks and does not resolve buffer credit issues on a direct dark fiber link; it is unnecessary here.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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