The correct answer is to configure a routed access layer with a link-state routing protocol such as IS-IS or OSPF. This is the most critical architectural consideration because the SD-Access underlay must provide pure IP connectivity between all fabric nodes—edge, control plane, and border—without relying on Spanning Tree Protocol. A routed access layer eliminates Layer 2 loops and STP dependencies, enabling fast convergence and a loop-free topology that directly supports the VXLAN tunnels used by the overlay. On the ENCOR 350-401 exam, this concept tests your understanding that the underlay is a simple, scalable IP transport, not a traditional switched network. A common trap is assuming the underlay can use VLANs or STP; remember that SD-Access requires every switch-to-switch link to be a routed Layer 3 interface. Memory tip: "Route the underlay, overlay the VXLAN—no STP, just IP."
CCNP Architecture Practice Question
This 350-401 practice question tests your understanding of architecture. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 global enterprise is transitioning from a traditional three-tier campus architecture to a software-defined access (SD-Access) fabric. Which architectural consideration is most critical for the underlay network?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure a routed access layer with a link-state routing protocol (IS-IS or OSPF).
In an SD-Access fabric, the underlay network must provide IP connectivity between all fabric devices (edge, control plane, border nodes) using a routed access layer with a link-state routing protocol like IS-IS or OSPF. This ensures fast convergence, loop-free topology, and support for the overlay's VXLAN tunnels. A routed access layer eliminates spanning-tree dependencies and aligns with the fabric's requirement for a simple, scalable IP-based transport.
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.
✓
Configure a routed access layer with a link-state routing protocol (IS-IS or OSPF).
Why this is correct
A routed underlay with IS-IS or OSPF is a key design requirement for SD-Access.
Related concept
Read the scenario before looking for a memorised answer.
✗
Implement PIM-SM for multicast routing in the underlay.
Why it's wrong here
Multicast is not required for the underlay; the overlay handles group communication.
✗
Preserve existing VLANs across the fabric to minimize changes.
Why it's wrong here
The underlay should be IP routed; VLANs are not used in the underlay.
✗
Deploy VRF-lite on all edge nodes to isolate tenants.
Why it's wrong here
VRF-lite is not a requirement; the overlay provides segmentation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that the underlay must support multicast (PIM) or preserve legacy VLANs, when in fact the underlay only needs unicast routing and the overlay handles all segmentation and multicast replication via head-end replication or native multicast.
Detailed technical explanation
How to think about this question
The underlay typically uses IS-IS as the IGP because it natively supports CLNS and can carry extended metrics for traffic engineering, though OSPF is also common. A key subtlety is that the underlay must support MTU of at least 1550 bytes (or 1600 with VXLAN overhead) to accommodate encapsulated packets; failure to configure jumbo frames or adjust MTU can cause blackholing. In real-world deployments, the underlay is often built with a spine-leaf topology using eBGP as an alternative to link-state protocols for better scalability in large fabrics.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Configure a routed access layer with a link-state routing protocol (IS-IS or OSPF). — In an SD-Access fabric, the underlay network must provide IP connectivity between all fabric devices (edge, control plane, border nodes) using a routed access layer with a link-state routing protocol like IS-IS or OSPF. This ensures fast convergence, loop-free topology, and support for the overlay's VXLAN tunnels. A routed access layer eliminates spanning-tree dependencies and aligns with the fabric's requirement for a simple, scalable IP-based transport.
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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Question Discussion
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