The answer is an authentication mismatch, specifically when EIGRP authentication is configured on one router but not on the other. This is the most likely cause of an EIGRP adjacency failure because EIGRP uses authentication to verify the integrity and source of hello packets; if one router sends authenticated hellos and the other does not, the receiving router silently discards those packets, preventing the neighbor relationship from forming even when K-values and the autonomous system number match perfectly. On the ENCOR 350-401 exam, this scenario tests your understanding that EIGRP authentication must be an all-or-nothing configuration between peers—a common trap is to focus only on mismatched K-values or AS numbers, forgetting that authentication breaks adjacency silently without any error messages. A helpful memory tip is to think of it as a "handshake lock": if one side locks the handshake with a key and the other doesn’t have it, they can never shake hands.
CCNP Infrastructure Practice Question
This 350-401 practice question tests your understanding of infrastructure. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 engineer is troubleshooting an EIGRP adjacency issue between two routers. The engineer verifies that both routers have the same K-values and autonomous system number. However, the adjacency does not form. Which configuration issue is most likely the cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Authentication is configured on one router but not on the other.
In EIGRP, authentication (MD5 or SHA) must be configured identically on both peers. If one router has authentication enabled and the other does not, the routers will reject each other's hello packets, preventing adjacency formation even if K-values and AS numbers match. This is a common misconfiguration that breaks neighbor relationships silently.
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.
✓
Authentication is configured on one router but not on the other.
One router has a loopback interface that is not advertised.
Why it's wrong here
Loopback interfaces are not required for EIGRP adjacency.
✗
The hello and hold timers do not match.
Why it's wrong here
EIGRP hello and hold timers do not need to match for adjacency to form.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that EIGRP requires matching hello and hold timers (like OSPF), but EIGRP is more tolerant; the real adjacency blocker is authentication mismatch, which is frequently overlooked when K-values and AS numbers are correct.
Detailed technical explanation
How to think about this question
EIGRP authentication uses a key chain with key IDs and passwords; the routers must share the same key ID and password, and the authentication mode (md5 or hmac-sha-256) must match. Under the hood, EIGRP appends an authentication TLV to hello packets; if the receiving router fails validation, it drops the packet and logs a 'authentication mismatch' error. In real-world scenarios, this often occurs after a password change on one router without updating the peer, causing silent adjacency loss.
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.
Infrastructure — This question tests Infrastructure — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Authentication is configured on one router but not on the other. — In EIGRP, authentication (MD5 or SHA) must be configured identically on both peers. If one router has authentication enabled and the other does not, the routers will reject each other's hello packets, preventing adjacency formation even if K-values and AS numbers match. This is a common misconfiguration that breaks neighbor relationships silently.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.