The answer is VXLAN overlay with policy-based forwarding (PBF) because it enables NFV service chaining by encapsulating traffic and steering it through a sequence of VNFs based on policies, allowing dynamic insertion of new VNFs without any physical re-cabling. In a virtualized environment where firewalls, routers, and WAN optimizers run on the same server, policy-based forwarding directs traffic between VNFs by matching packet attributes to forwarding rules, while VXLAN provides the overlay that isolates and transports the chained flows across the underlay network. On the ENCOR 350-401 exam, this tests your understanding of how overlay technologies decouple service chains from physical topology, with a common trap being to choose VLAN-based segmentation or static routing, which lack the flexibility for dynamic VNF insertion. Remember the mnemonic "VX-PBF" as "Virtual eXtensible Policy-Based Flow" to recall that the overlay provides the fabric and PBF provides the steering logic for service chaining.
350-401 Enterprise Network Design Practice Question
This 350-401 practice question tests your understanding of enterprise network design. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 service provider is deploying NFV to host virtual network functions (VNFs) such as firewalls, routers, and WAN optimizers on a single server. The design must support service chaining, where traffic flows through multiple VNFs in a specific order, and must allow dynamic insertion of new VNFs without re-cabling. Which technology should be used to implement the service chain?
Why wrong: VLANs are limited to Layer 2 and require physical reconfiguration for changes; they do not support dynamic service chaining across multiple VNFs.
B
VXLAN overlay with policy-based forwarding to direct traffic through VNFs
VXLAN enables flexible, scalable service chaining by encapsulating traffic and steering it through VNFs based on policies.
C
Static routing between VNFs using dedicated interfaces
Why wrong: Static routing requires manual configuration and does not support dynamic insertion or reordering of VNFs.
D
MPLS L3VPN between VNFs
Why wrong: MPLS L3VPN is designed for WAN connectivity, not for service chaining within a single NFV host.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
VXLAN overlay with policy-based forwarding to direct traffic through VNFs
VXLAN overlay with policy-based forwarding (PBF) is the correct choice because it enables service chaining by encapsulating traffic and steering it through a sequence of VNFs based on policies, without requiring physical re-cabling. This allows dynamic insertion of new VNFs by simply updating the forwarding policies in the overlay, which is essential for NFV environments where VNFs are hosted on the same server and must be chained flexibly.
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.
✗
VLAN trunking between VNFs on the same hypervisor
Why it's wrong here
VLANs are limited to Layer 2 and require physical reconfiguration for changes; they do not support dynamic service chaining across multiple VNFs.
✓
VXLAN overlay with policy-based forwarding to direct traffic through VNFs
Why this is correct
VXLAN enables flexible, scalable service chaining by encapsulating traffic and steering it through VNFs based on policies.
Related concept
Read the scenario before looking for a memorised answer.
✗
Static routing between VNFs using dedicated interfaces
Why it's wrong here
Static routing requires manual configuration and does not support dynamic insertion or reordering of VNFs.
✗
MPLS L3VPN between VNFs
Why it's wrong here
MPLS L3VPN is designed for WAN connectivity, not for service chaining within a single NFV host.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse VLAN trunking (Option A) as sufficient for service chaining, but VLANs only provide segmentation, not the policy-based traffic steering required to enforce a specific ordered sequence of VNFs.
Detailed technical explanation
How to think about this question
VXLAN overlay uses a 24-bit VNI to create isolated Layer 2 segments over a Layer 3 underlay, and policy-based forwarding (PBF) leverages match criteria (e.g., source/destination IP, port) to redirect traffic to a specific next-hop VNF, enabling service chaining. In a real-world NFV deployment, the VNFs are connected to a virtual switch (e.g., Open vSwitch) that applies PBF rules to forward traffic between VNFs in the chain, and new VNFs can be inserted by updating the flow table without any physical cabling changes. This approach is standardized in RFC 7348 for VXLAN and is commonly used in Cisco NFVIS and OpenStack environments.
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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
Enterprise Network Design — This question tests Enterprise Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: VXLAN overlay with policy-based forwarding to direct traffic through VNFs — VXLAN overlay with policy-based forwarding (PBF) is the correct choice because it enables service chaining by encapsulating traffic and steering it through a sequence of VNFs based on policies, without requiring physical re-cabling. This allows dynamic insertion of new VNFs by simply updating the forwarding policies in the overlay, which is essential for NFV environments where VNFs are hosted on the same server and must be chained flexibly.
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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