350-401 Infrastructure Practice Question
A network engineer is implementing a VXLAN overlay over an existing Layer 3 campus network. The requirement is to carry Layer 2 frames across the Layer 3 underlay. Which protocol is used to encapsulate the original Layer 2 frame for transport across the IP network?
⚠ Common exam trap
The trap here is assuming that any tunneling protocol can carry Layer 2 over Layer 3, when VXLAN specifically uses UDP encapsulation and a VNI, not GRE or MPLS.
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
✓
VXLAN with a UDP header
VXLAN encapsulates the original Layer 2 frame in a MAC-in-UDP format. The outer IP and UDP headers allow the encapsulated frame to be routed across a Layer 3 underlay, which is exactly what the scenario requires. Other encapsulation methods either need a different underlay or do not provide the VXLAN VNI and VTEP behavior.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
802.1Q tagging on the underlay
Why it's wrong here
802.1Q tags are added to Ethernet frames on trunk links and are not routable across a Layer 3 underlay. The underlay in this scenario is Layer 3, so 802.1Q alone cannot transport the Layer 2 frame between VTEPs. It is a Layer 2 mechanism.
- ✗
MPLS label stack with a Layer 2 VPN
Why it's wrong here
MPLS Layer 2 VPNs can carry Layer 2 frames, but they require an MPLS-enabled underlay. The scenario describes an IP underlay and VXLAN, which uses UDP/IP encapsulation rather than MPLS labels. This option does not use the VXLAN data plane.
- ✓
VXLAN with a UDP header
Why this is correct
VXLAN encapsulates the original Layer 2 frame inside a MAC-in-UDP header. The 8-byte VXLAN header, UDP header, and outer IP header allow the frame to be routed across a Layer 3 underlay. This directly satisfies the scenario requirement to carry Layer 2 frames over an IP network.
- ✗
GRE with a protocol type of 0x6558
Why it's wrong here
GRE transparent Ethernet bridging uses protocol type 0x6558, but it is not the encapsulation used by VXLAN. GRE is a separate tunneling protocol and would require its own configuration; it does not provide the VXLAN VNI or the scalability features the scenario implies.
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
Go deeper
Related to this question
Learn chapter
VLANs and Spanning Tree Protocol Concepts
Key term
VXLAN
VXLAN is a network overlay technology that encapsulates Layer 2 Ethernet frames in UDP packets to extend VLANs across Layer 3 networks.
Key term
Cisco Virtual Topology System
Cisco Virtual Topology System is a software-defined networking solution that creates and manages virtual network overlays across physical and virtual infrastructure for enterprise networks.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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.