Question 304 of 2,015

EIGRP →easyMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the EIGRP authentication mismatch prevents neighbor adjacency because R1 has MD5 authentication enabled while R2 does not, causing R1 to discard R2’s unauthenticated hello packets. EIGRP requires both sides of a link to use identical authentication settings—including the same mode and key—for the hello and update exchange to be accepted; when one router sends authenticated hellos and the other sends plain hellos, the receiving router with authentication enabled rejects the mismatched packets, breaking the two-way adjacency. On the ENCOR 350-401 exam, this scenario tests your understanding of EIGRP neighbor formation prerequisites and the common trap of assuming a one-way adjacency is valid—remember that a neighbor state of INIT on one side and no neighbor on the other is a classic sign of authentication or K-value mismatch. A helpful memory tip: “Authenticate both sides, or the neighbor hides.”

350-401 EIGRP Practice Question

This 350-401 practice question tests your understanding of eigrp. 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 issue where two routers, R1 and R2, are directly connected. R1 shows an EIGRP adjacency with R2, but R2 does not show an adjacency with R1. The engineer checks the interface configurations and finds that R1 has 'ip authentication mode eigrp 1 md5' and 'ip authentication key-chain eigrp 1 MYKEY' configured, while R2 has no authentication configured. What is the most likely 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.

Question 1easymultiple choice

Study the full EIGRP explanation →

A
R1 has authentication configured, but R2 does not, so R1 will reject R2's hello packets, and no adjacency forms.
Correct. EIGRP authentication requires both sides to have matching authentication. Since R2 does not have authentication, R1 will discard R2's hello packets, preventing adjacency.
B
R2 will automatically learn the authentication key from R1 and form an adjacency.
Why wrong: Incorrect. Authentication keys are not learned dynamically; they must be configured manually.
C
R1 will form an adjacency with R2 because authentication is optional.
Why wrong: Incorrect. If authentication is configured on one side, it is enforced; the other side must also have authentication configured.
D
The adjacency will form but only for routes that are not authenticated.
Why wrong: Incorrect. Authentication applies to all EIGRP packets; if it fails, no adjacency forms.

Full breakdown with real-world context →

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

✓

R1 has authentication configured, but R2 does not, so R1 will reject R2's hello packets, and no adjacency forms.

EIGRP authentication is configured per interface and must match on both sides for an adjacency to form. R1 has MD5 authentication enabled, so it will include the authentication data in its hello packets and will reject any hello packets received from R2 that lack the correct authentication. Since R2 has no authentication configured, its hello packets are sent without authentication data, causing R1 to discard them. Consequently, R1 sees R2 as a neighbor (because it receives R2's unauthenticated hellos and may still attempt to form adjacency depending on implementation), but R2 never receives a valid hello from R1 (because R1's hellos are authenticated and R2 does not process the authentication field), so R2 does not establish an adjacency with R1.

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.

✓
R1 has authentication configured, but R2 does not, so R1 will reject R2's hello packets, and no adjacency forms.
Why this is correct
Correct. EIGRP authentication requires both sides to have matching authentication. Since R2 does not have authentication, R1 will discard R2's hello packets, preventing adjacency.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
R2 will automatically learn the authentication key from R1 and form an adjacency.
Why it's wrong here
Incorrect. Authentication keys are not learned dynamically; they must be configured manually.
✗
R1 will form an adjacency with R2 because authentication is optional.
Why it's wrong here
Incorrect. If authentication is configured on one side, it is enforced; the other side must also have authentication configured.
✗
The adjacency will form but only for routes that are not authenticated.
Why it's wrong here
Incorrect. Authentication applies to all EIGRP packets; if it fails, no adjacency forms.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that authentication is optional or that a router can learn keys dynamically; the trap here is assuming that an adjacency can form unidirectionally when authentication is mismatched, when in fact EIGRP requires matching authentication parameters for bidirectional neighbor discovery.

Detailed technical explanation

How to think about this question

EIGRP uses MD5 or SHA-256 hashing (via key chains) to authenticate packets. The authentication data is carried in the EIGRP packet header as a Type-Length-Value (TLV) field. If the receiving router has authentication enabled, it computes the hash of the received packet using its configured key and compares it to the hash in the packet; if they do not match (or if the authentication TLV is missing), the packet is silently dropped. This behavior is defined in RFC 7868. In a real-world scenario, a misconfigured key chain (e.g., wrong key ID or key string) would cause a similar one-way adjacency issue.

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.

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FAQ

Questions learners often ask

What does this 350-401 question test?

EIGRP — This question tests EIGRP — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: R1 has authentication configured, but R2 does not, so R1 will reject R2's hello packets, and no adjacency forms. — EIGRP authentication is configured per interface and must match on both sides for an adjacency to form. R1 has MD5 authentication enabled, so it will include the authentication data in its hello packets and will reject any hello packets received from R2 that lack the correct authentication. Since R2 has no authentication configured, its hello packets are sent without authentication data, causing R1 to discard them. Consequently, R1 sees R2 as a neighbor (because it receives R2's unauthenticated hellos and may still attempt to form adjacency depending on implementation), but R2 never receives a valid hello from R1 (because R1's hellos are authenticated and R2 does not process the authentication field), so R2 does not establish an adjacency with R1.

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.

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