The answer is that host memory pressure causing the hypervisor to balloon or swap memory from the vFW is the most likely cause of the packet loss. When the NFVIS host runs low on physical memory, the hypervisor reclaims RAM from virtual machines through mechanisms like memory ballooning or transparent page sharing, which forcibly reduces the vFW’s available buffers and operating system memory. This forces the vFW to drop incoming packets because it cannot allocate the necessary memory for packet processing, even though its CPU usage remains low—a classic sign that the bottleneck is memory, not processing power. On the ENCOR 350-401 exam, this scenario tests your understanding of virtualization overhead and resource contention on NFVIS hosts; a common trap is to blame the vFW’s CPU or configuration when the root cause is host-level memory exhaustion. Remember the mnemonic: “Low CPU, high host RAM? Ballooning’s the jam.”
350-401 Network Function Virtualization Practice Question
This 350-401 practice question tests your understanding of network function virtualization. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 troubleshooting a performance issue with a virtual firewall (vFW) running on a Cisco NFVIS host. The vFW is experiencing high packet loss during peak traffic. The engineer checks the NFVIS monitoring dashboard and sees that the vFW's CPU usage is low, but the host's memory usage is high. What is the most likely cause of the packet loss?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The host is under memory pressure, causing the hypervisor to swap or balloon memory from the vFW.
When the NFVIS host experiences high memory pressure, the hypervisor may reclaim memory from virtual machines (VMs) using mechanisms such as ballooning or swapping. This reduces the memory available to the vFW, causing it to drop packets because its packet buffers or operating system memory are forcibly reclaimed. The vFW's CPU remains low because the bottleneck is memory, not processing power.
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.
✗
The vFW is CPU-bound, but the monitoring is inaccurate.
Why it's wrong here
Incorrect because the vFW's CPU usage is low, so it is not CPU-bound.
✗
The host's CPU is oversubscribed, causing vCPU starvation.
Why it's wrong here
Incorrect because the host's memory is high, not CPU; CPU starvation would show high vFW CPU usage.
✓
The host is under memory pressure, causing the hypervisor to swap or balloon memory from the vFW.
Why this is correct
Correct because high host memory usage can lead to memory reclaiming, which impacts vFW performance and causes packet loss.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The vFW's packet buffer is exhausted, but the monitoring does not show it.
Why it's wrong here
Incorrect because packet buffer exhaustion would likely be reflected in vFW logs, and the host memory issue is a more direct cause.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between CPU and memory bottlenecks in virtualized environments, where candidates mistakenly assume high packet loss must be CPU-related, ignoring that memory pressure from the hypervisor can cause the vFW to lose packets even when its CPU is idle.
Trap categories for this question
Command / output trap
Incorrect because the host's memory is high, not CPU; CPU starvation would show high vFW CPU usage.
Detailed technical explanation
How to think about this question
In NFVIS, the hypervisor (KVM-based) uses kernel same-page merging (KSM) and balloon drivers to manage memory. When host memory is overcommitted, the balloon driver inflates within the guest VM, reducing its available memory and forcing the guest OS to swap or drop packets. This is a common issue in virtualized network functions where memory guarantees are not set, and it can be diagnosed by checking the 'virtio_balloon' statistics or using 'show system resources' on the NFVIS CLI.
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 350-401 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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Network Function Virtualization — This question tests Network Function Virtualization — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The host is under memory pressure, causing the hypervisor to swap or balloon memory from the vFW. — When the NFVIS host experiences high memory pressure, the hypervisor may reclaim memory from virtual machines (VMs) using mechanisms such as ballooning or swapping. This reduces the memory available to the vFW, causing it to drop packets because its packet buffers or operating system memory are forcibly reclaimed. The vFW's CPU remains low because the bottleneck is memory, not processing power.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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