The correct answer is Priority Flow Control (PFC) with a no-drop queue for iSCSI. This is because PFC, defined by IEEE 802.1Qbb, operates on a per-priority basis, using pause frames to prevent buffer overflow specifically for the iSCSI traffic class, thereby guaranteeing the lossless behavior that storage protocols demand. On the Cisco DCCOR 350-601 exam, this question tests your understanding of data center QoS architectures, where a common trap is confusing PFC with global flow control (IEEE 802.3x) or assuming standard tail-drop queuing is sufficient for storage. Remember that iSCSI requires a dedicated no-drop queue, and PFC is the only mechanism that provides per-priority lossless handling on Nexus 9000 switches. A useful memory tip: "PFC pauses per priority, not the port" — so for iSCSI, you configure a no-drop queue and map it to a specific CoS value.
350-601 Network Practice Question
This 350-601 practice question tests your understanding of 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.
A network engineer is implementing QoS on a Nexus 9000 switch. The requirement is to prioritize storage traffic (iSCSI) and ensure lossless behavior. Which queuing strategy should be applied to the egress interface?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Priority Flow Control (PFC) with a no-drop queue for iSCSI.
Option C is correct because Priority Flow Control (PFC) is the IEEE 802.1Qbb mechanism designed to provide lossless behavior for specific traffic classes, such as iSCSI storage traffic, on Nexus 9000 switches. By creating a no-drop queue for iSCSI, PFC uses pause frames on a per-priority basis to prevent buffer overflow, ensuring zero packet loss required by storage protocols.
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.
✗
Tail drop with DSCP-based classification.
Why it's wrong here
Tail drop can cause packet loss, which is unacceptable for iSCSI.
✗
Weighted Round Robin (WRR) with three queues.
Why it's wrong here
WRR does not provide lossless behavior.
✓
Priority Flow Control (PFC) with a no-drop queue for iSCSI.
Why this is correct
PFC enables lossless Ethernet by pausing traffic when buffers are full.
Related concept
Read the scenario before looking for a memorised answer.
✗
Policing at the ingress and marking at the egress.
Why it's wrong here
Policing drops packets, which can cause retransmissions.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that any queuing or scheduling algorithm (like WRR or tail drop) can provide lossless behavior, but the trap here is that only PFC with a dedicated no-drop queue satisfies the strict no-loss requirement for storage traffic like iSCSI or FCoE.
Detailed technical explanation
How to think about this question
PFC operates by dividing a physical link into eight virtual lanes (priorities) using 802.1p CoS values; when a no-drop queue (e.g., CoS 3 for iSCSI) approaches congestion, the switch sends a pause frame to the upstream device to halt transmission for that priority only, while other priorities continue unimpeded. In a real-world scenario, if PFC is misconfigured or a deadlock occurs (e.g., due to a loop), it can cause head-of-line blocking across the fabric, which is why proper buffer management and flow control tuning are critical.
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 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
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 — This question tests Network — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Priority Flow Control (PFC) with a no-drop queue for iSCSI. — Option C is correct because Priority Flow Control (PFC) is the IEEE 802.1Qbb mechanism designed to provide lossless behavior for specific traffic classes, such as iSCSI storage traffic, on Nexus 9000 switches. By creating a no-drop queue for iSCSI, PFC uses pause frames on a per-priority basis to prevent buffer overflow, ensuring zero packet loss required by storage protocols.
What should I do if I get this 350-601 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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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