- A
Apply classification and marking on the leaf switches at ingress, and queuing policies on egress interfaces of both leaf and spine switches.
Ingress classification at the leaf marks traffic; egress queuing on leaf and spine ensures consistent PHB across the fabric.
- B
Apply all QoS policies only on the spine switches, since they handle inter-leaf traffic.
Why wrong: Spine switches do not see traffic from servers directly; classification must occur at the leaf where traffic enters.
- C
Configure QoS only on the default gateway router, which is upstream of the leaf-spine fabric.
Why wrong: This would not affect traffic within the fabric, failing to prioritize iSCSI over backup inside the data center.
- D
Use a single QoS policy on all interfaces with default settings, relying on hardware buffers.
Why wrong: Default settings do not differentiate traffic; iSCSI and backup would compete, causing potential performance issues.
Quick Answer
The correct answer is to apply classification and marking on the leaf switches at ingress, and queuing policies on egress interfaces of both leaf and spine switches. This is because in a data center leaf-spine QoS classification and queuing design, the leaf switch is the trust boundary where traffic first enters the fabric, making it the only logical point to inspect and mark flows like iSCSI, backup, and real-time applications before they traverse the spine. Queuing must then be applied on all egress interfaces—both leaf-to-spine and spine-to-leaf—to enforce priority and low latency across the entire east-west path, preventing congestion from degrading time-sensitive traffic. On the ENCOR 350-401 exam, this tests your understanding that classification happens once at the edge, while queuing must be consistent throughout the fabric; a common trap is applying queuing only on the leaf egress, forgetting that spine egress also needs policies to protect traffic on the return path. Remember the mnemonic: Mark at the Leaf, Queue on Every Egress.
350-401 QoS Architecture Practice Question
This 350-401 practice question tests your understanding of qos architecture. 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.
An enterprise is deploying a leaf-spine architecture in its data center to support high-bandwidth east-west traffic. The design must include QoS to prioritize storage replication traffic (iSCSI) over backup traffic, while ensuring low latency for real-time applications. Where should the architect apply QoS classification and queuing policies in this topology?
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
Apply classification and marking on the leaf switches at ingress, and queuing policies on egress interfaces of both leaf and spine switches.
In a leaf-spine architecture, QoS classification and marking must occur at the ingress of leaf switches (where traffic enters the fabric) to identify iSCSI, backup, and real-time flows. Queuing policies must be applied on egress interfaces of both leaf and spine switches to manage congestion and prioritize latency-sensitive traffic across the entire path, ensuring end-to-end QoS for east-west traffic.
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.
- ✓
Apply classification and marking on the leaf switches at ingress, and queuing policies on egress interfaces of both leaf and spine switches.
Why this is correct
Ingress classification at the leaf marks traffic; egress queuing on leaf and spine ensures consistent PHB across the fabric.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Apply all QoS policies only on the spine switches, since they handle inter-leaf traffic.
Why it's wrong here
Spine switches do not see traffic from servers directly; classification must occur at the leaf where traffic enters.
- ✗
Configure QoS only on the default gateway router, which is upstream of the leaf-spine fabric.
Why it's wrong here
This would not affect traffic within the fabric, failing to prioritize iSCSI over backup inside the data center.
- ✗
Use a single QoS policy on all interfaces with default settings, relying on hardware buffers.
Why it's wrong here
Default settings do not differentiate traffic; iSCSI and backup would compete, causing potential performance issues.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that QoS policies should be applied only at the core or spine layer, but the correct approach requires classification at the edge (leaf ingress) and queuing on all egress interfaces to ensure end-to-end treatment across the fabric.
Detailed technical explanation
How to think about this question
Under the hood, Cisco leaf-spine designs often use MQC (Modular QoS CLI) to classify traffic based on DSCP or CoS values, with marking applied at ingress using 'set dscp' or 'set cos' commands. Queuing on egress interfaces leverages class-maps and policy-maps to assign traffic to priority queues (e.g., for real-time applications) or bandwidth queues (e.g., for iSCSI), while backup traffic may be placed in a scavenger queue with minimal bandwidth. In a real-world scenario, misclassification of iSCSI as best-effort can cause timeouts during storage replication, while failing to prioritize VoIP or video can degrade user experience.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
Questions learners often ask
What does this 350-401 question test?
QoS Architecture — This question tests QoS Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Apply classification and marking on the leaf switches at ingress, and queuing policies on egress interfaces of both leaf and spine switches. — In a leaf-spine architecture, QoS classification and marking must occur at the ingress of leaf switches (where traffic enters the fabric) to identify iSCSI, backup, and real-time flows. Queuing policies must be applied on egress interfaces of both leaf and spine switches to manage congestion and prioritize latency-sensitive traffic across the entire path, ensuring end-to-end QoS for east-west traffic.
What should I do if I get this 350-401 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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Last reviewed: Jun 24, 2026
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