Quick Answer
IS-IS is the correct choice because it is a link-state routing protocol that provides fast convergence, is highly scalable, and is the most commonly recommended routing protocol for the underlay of an SD-Access fabric. As a link-state protocol, IS-IS builds a complete topology database using SPF calculations, enabling rapid rerouting around failures without the periodic updates or hop-count limitations of distance-vector protocols. On the ENCOR 350-401 exam, this question tests your understanding of SD-Access underlay design principles, often appearing in a scenario where you must distinguish IS-IS from OSPF or EIGRP. A common trap is choosing OSPF because it is also link-state, but Cisco SD-Access designs specifically favor IS-IS for its native support of CLNS, its ability to handle large flat Layer 2 domains with minimal overhead, and its simpler configuration in fabric environments. Remember the memory tip: “IS-IS for SD-Access underlay” — think of the double “I” in both names to link the protocol to the fabric’s underlay routing requirement.
This 350-401 practice question tests your understanding of sd-access 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 network team is designing the underlay for an SD-Access fabric. The design must use a routing protocol that supports fast convergence and is commonly recommended for the fabric underlay. Which routing protocol should be used?
IS-IS
IS-IS is the preferred underlay routing protocol for SD-Access fabric.
RIP
Why wrong: RIP is outdated and not suitable for modern fabric underlays.
EIGRP
Why wrong: EIGRP is a Cisco proprietary protocol but not recommended for SD-Access underlay.
BGP
Why wrong: BGP is used for overlay control plane (LISP) but not typically for the underlay.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
IS-IS
IS-IS is the correct choice because it is a link-state routing protocol that provides fast convergence, is highly scalable, and is the most commonly recommended routing protocol for the underlay of an SD-Access fabric. Cisco SD-Access designs frequently use IS-IS to support the fabric's control plane and data plane requirements, leveraging its ability to handle large, flat network topologies with minimal overhead.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
IS-IS
Why this is correct
IS-IS is the preferred underlay routing protocol for SD-Access fabric.
Related concept
Read the scenario before looking for a memorised answer.
RIP
Why it's wrong here
RIP is outdated and not suitable for modern fabric underlays.
EIGRP
Why it's wrong here
EIGRP is a Cisco proprietary protocol but not recommended for SD-Access underlay.
BGP
Why it's wrong here
BGP is used for overlay control plane (LISP) but not typically for the underlay.
Common exam traps
Cisco often tests the misconception that EIGRP is the best choice for fast convergence in Cisco-centric designs, but for SD-Access underlay, the recommended protocol is IS-IS due to its open standard nature and alignment with Cisco's validated fabric architecture.
Detailed technical explanation
IS-IS uses the SPF (Shortest Path First) algorithm, which recalculates routes quickly upon topology changes, and it operates directly over Layer 2, making it efficient for large-scale fabrics. In SD-Access, the underlay typically uses IS-IS with wide metrics to support higher bandwidth links and fast IGP convergence, while the overlay uses LISP for endpoint mobility and VXLAN for data encapsulation. A real-world scenario is a campus fabric with hundreds of edge nodes where IS-IS's hierarchical design (Level 1/Level 2) helps isolate routing updates and reduce CPU load.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 350-401 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
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FAQ
SD-Access Architecture — This question tests SD-Access Architecture — Read the scenario before looking for a memorised answer..
The correct answer is: IS-IS — IS-IS is the correct choice because it is a link-state routing protocol that provides fast convergence, is highly scalable, and is the most commonly recommended routing protocol for the underlay of an SD-Access fabric. Cisco SD-Access designs frequently use IS-IS to support the fabric's control plane and data plane requirements, leveraging its ability to handle large, flat network topologies with minimal overhead.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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.
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