350-401 Architecture Practice Question
A network architect is designing a campus network that must support thousands of endpoints with a fabric overlay. The design requires that the underlay provide fast convergence and equal-cost multipathing without running a separate routing protocol in the overlay. Which underlay routing protocol should be selected to meet these requirements?
⚠ Common exam trap
The trap here is assuming any protocol with ECMP support is equally suitable as a fabric underlay, when IS-IS is specifically recommended for SD-Access due to its convergence and integration characteristics.
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
✓
Intermediate System-to-Intermediate System (IS-IS)
Cisco SD-Access recommends IS-IS as the underlay routing protocol because it provides fast convergence, scalable ECMP, and clean separation from the LISP/VXLAN overlay. OSPF, EIGRP, and BGP can provide routing but are not the standard underlay choice for large fabric deployments and may add complexity or converge more slowly in this context.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Intermediate System-to-Intermediate System (IS-IS)
Why this is correct
IS-IS is the recommended underlay routing protocol for Cisco SD-Access fabrics. It provides fast convergence, supports ECMP, and scales well in large campus deployments. It operates independently of the LISP/VXLAN overlay, allowing the underlay to focus on IP reachability. This directly meets the requirements for fast convergence and multipathing without overlay routing, making it the correct selection.
- ✗
Enhanced Interior Gateway Routing Protocol (EIGRP)
Why it's wrong here
EIGRP offers fast convergence and ECMP, but it is not the standard underlay choice for Cisco SD-Access. It is a Cisco-proprietary protocol and, while supported in some designs, does not integrate with fabric automation as cleanly as IS-IS. For a large-scale fabric requiring best-practice alignment, EIGRP would not be the recommended underlay, so this option is not ideal.
- ✗
Border Gateway Protocol (BGP)
Why it's wrong here
BGP is typically used at the fabric border for external connectivity, not as the underlay routing protocol within the campus fabric. It converges more slowly than IS-IS in this context and is not optimized for the dense ECMP requirements of the underlay. Using BGP as the underlay would not meet the fast convergence and simplicity goals, making this option incorrect.
- ✗
Open Shortest Path First (OSPF) with multiple areas
Why it's wrong here
OSPF can provide fast convergence and ECMP, but it is not the recommended underlay for Cisco SD-Access fabrics. It adds complexity and does not integrate as seamlessly with the fabric automation and LISP/VXLAN overlay as IS-IS does. While technically possible, it does not align with the design best practice for large-scale fabric underlays, so this option is not the best choice.
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
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.
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.
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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.