350-401 Infrastructure Practice Question
A network engineer is deploying a new Cisco SD-Access fabric. The design includes underlay and overlay networks. Which two statements accurately describe the underlay network in a Cisco SD-Access fabric? (Choose two.)
⚠ Common exam trap
Many exam-takers confuse underlay and overlay responsibilities; VXLAN encapsulation and LISP mapping are overlay functions, not underlay.
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
✓
The underlay provides a loop-free topology using a routing protocol and does not rely on Spanning Tree Protocol.
In Cisco SD-Access, the underlay provides IP reachability between fabric nodes using a routing protocol such as IS-IS or OSPF, and it is a loop-free Layer 3 network that does not rely on Spanning Tree Protocol. VXLAN encapsulation and LISP mapping are overlay functions. Static routing is not the typical underlay approach.
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 underlay encapsulates user traffic in VXLAN to provide overlay connectivity.
Why it's wrong here
VXLAN encapsulation is a function of the overlay network, not the underlay. The underlay forwards the encapsulated packets based on the outer IP header. The underlay itself does not perform VXLAN encapsulation; that is done by the fabric edge nodes. Therefore, this statement is incorrect.
- ✓
The underlay provides a loop-free topology using a routing protocol and does not rely on Spanning Tree Protocol.
Why this is correct
The underlay is typically a Layer 3 routed network that uses a routing protocol to ensure loop-free paths. It does not rely on Spanning Tree Protocol, which is a Layer 2 loop prevention mechanism. This is a correct characteristic of the SD-Access underlay.
- ✗
The underlay is responsible for mapping endpoint IP addresses to fabric locators (RLOCs).
Why it's wrong here
Mapping endpoint IP addresses to fabric locators (RLOCs) is the function of the LISP control plane, which is part of the overlay. The underlay only provides IP connectivity between the RLOCs. Therefore, this statement is incorrect.
- ✗
The underlay uses a static routing protocol to simplify configuration and reduce overhead.
Why it's wrong here
While static routing can be used in small deployments, Cisco SD-Access underlay typically uses a dynamic routing protocol like IS-IS or OSPF for scalability and automatic failover. Static routing is not the standard recommendation for a fabric underlay, especially in larger networks, so this statement is not accurate.
- ✓
The underlay uses a routing protocol such as IS-IS or OSPF to provide reachability between fabric nodes.
Why this is correct
The underlay network in SD-Access is responsible for providing IP reachability between all fabric nodes (e.g., border nodes, control plane nodes, edge nodes). It typically uses a dynamic routing protocol like IS-IS or OSPF to exchange routes. This is a correct statement about the underlay.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
SDN Controllers and Cisco ACI
Key term
Fabric Fundamentals
Fabric Fundamentals is the set of core concepts behind a network fabric, where switches and routers form a single logical system that simplifies traffic forwarding and automation.
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.