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CCNP Practice Question: Deploying a virtualized router (CSR1000v) on…
A company is deploying a virtualized router (CSR1000v) on VMware vSphere. The VNF must support high throughput and low latency. Which vSphere configuration option should the architect select to optimize network performance?
⚠ Common exam trap
Cisco often tests the misconception that VMXNET3 is the best performance option for all VNFs, but the trap here is that SR-IOV is required when the question explicitly demands 'high throughput and low latency' because it eliminates hypervisor overhead.
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
✓
Enable SR-IOV on the physical NIC and assign virtual functions to the VM.
SR-IOV (Single Root I/O Virtualization) allows a physical NIC to present multiple virtual functions (VFs) directly to a VM, bypassing the hypervisor's virtual switch. This reduces latency and CPU overhead, making it ideal for high-throughput, low-latency VNFs like the CSR1000v. Option B is correct because SR-IOV provides near-native performance by allowing the VM to directly access the NIC hardware.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use the default e1000 NIC driver.
Why it's wrong here
The e1000 driver emulates a legacy Intel 82545EM Ethernet controller entirely in software, exposing a virtual PCI device to the guest. Every frame transmission and interrupt traverses the hypervisor's emulation layer, which consumes significant host CPU and incurs high per-packet latency. It also lacks multi-queue support and advanced offloads, making it unsuitable for NFV data-plane workloads that demand high throughput and low jitter.
- ✓
Enable SR-IOV on the physical NIC and assign virtual functions to the VM.
Why this is correct
SR-IOV (Single Root I/O Virtualization) partitions a physical NIC into multiple Virtual Functions (VFs), each with dedicated hardware queues, interrupts, and DMA resources. When a VF is directly assigned to a VM using PCI passthrough, the guest driver communicates with the NIC hardware without hypervisor mediation in the data path, yielding near-native throughput and latencies. This is the correct choice for NFV because it bypasses the virtual switch and most of the hypervisor's I/O stack, though it requires SR-IOV-capable hardware and sacrifices some features like live migration.
- ✗
Use VMXNET3 paravirtualized NIC.
Why it's wrong here
VMXNET3 is a paravirtualized NIC that improves performance over e1000 by using shared memory rings and a driver that cooperates with the hypervisor's ESXi networking stack. However, the hypervisor still participates in packet forwarding, interrupt delivery, and each I/O operation, adding CPU overhead compared to direct hardware access. It supports multi-queue and modern offloads, but under heavy NFV data-plane traffic it still bottlenecks at the hypervisor's software switch, so it is inferior to SR-IOV.
- ✗
Configure the VM with multiple vCPUs and large memory.
Why it's wrong here
Adding vCPUs and memory is a general resource scaling strategy that can help if the VM is CPU- or memory-constrained, but it does not address the root cause of poor network I/O—the emulated or paravirtualized device path that serializes packet processing and injects per-packet overhead. Without mechanisms like multi-queue RSS or direct device assignment, extra vCPUs simply spin on locks waiting for the same slow I/O interface. Therefore, this approach fails to deliver the line-rate throughput and deterministic latency that NFV workloads require.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.