Question 165 of 2,015

EIGRP →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the tunnel interface is missing the ip eigrp command under its configuration. Without this command, EIGRP is not enabled on the DMVPN tunnel interface, so it cannot send or process Hello packets, which are essential for forming and maintaining neighbor adjacencies. Even if NHRP settings like network-id and nhs are correctly configured, EIGRP will remain silent on that interface, explaining why the adjacency forms only on the point-to-point link where the command is present. On the ENCOR 350-401 exam, this scenario tests your understanding that EIGRP must be explicitly enabled on each interface where you want neighbor relationships, and it is a common trap to assume that a tunnel interface inherits EIGRP configuration from the physical interface or subinterface. A useful memory tip: “No ip eigrp, no hello, no adjacency” — always check that the EIGRP process is activated under the specific tunnel interface.

350-401 EIGRP Practice Question

This 350-401 practice question tests your understanding of eigrp. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.

An engineer configures EIGRP on a new router in a DMVPN network. The router has a single physical interface with two subinterfaces: one for the DMVPN tunnel and one for a direct point-to-point link to a hub router. The engineer notices that EIGRP adjacencies form only on the point-to-point link, not on the DMVPN tunnel. The tunnel interface is configured with ip nhrp network-id 1 and ip nhrp nhs 10.1.1.1. What is the most likely reason?

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 1hardmultiple choice

Study the full EIGRP explanation →

A
The tunnel interface is not configured with the 'ip nhrp map' command for the hub router.
Why wrong: Incorrect. The 'ip nhrp map' command is used for static mapping, but NHRP dynamically learns mappings in a typical DMVPN. The adjacency issue is more likely due to EIGRP not being enabled on the tunnel.
B
The tunnel interface is not configured with the 'ip eigrp' command under the interface configuration.
Correct. For EIGRP to form an adjacency over the tunnel interface, the interface must be included in the EIGRP process, typically with 'ip eigrp <as-number>' under the interface. Without it, EIGRP will not send or receive hello packets on that interface.
C
The DMVPN tunnel is using a different autonomous system number than the point-to-point link.
Why wrong: Incorrect. If the autonomous system numbers differ, EIGRP adjacencies would not form on any interface. The scenario states adjacencies form on the point-to-point link, so AS numbers match.
D
The tunnel interface is in a different VRF than the point-to-point link.
Why wrong: Incorrect. While VRF mismatch can prevent adjacencies, the scenario does not mention VRF configuration. The most common cause is missing EIGRP configuration on the tunnel interface.

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

✓

The tunnel interface is not configured with the 'ip eigrp' command under the interface configuration.

Option B is correct because the most common reason for EIGRP not forming an adjacency over a tunnel interface is the absence of the 'ip eigrp' command under that interface. Without this command, EIGRP is not enabled on the tunnel, so it will not send or process Hello packets, preventing adjacency formation even if NHRP and tunnel parameters are correctly configured.

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.

✗
The tunnel interface is not configured with the 'ip nhrp map' command for the hub router.
Why it's wrong here
Incorrect. The 'ip nhrp map' command is used for static mapping, but NHRP dynamically learns mappings in a typical DMVPN. The adjacency issue is more likely due to EIGRP not being enabled on the tunnel.
✓
The tunnel interface is not configured with the 'ip eigrp' command under the interface configuration.
Why this is correct
Correct. For EIGRP to form an adjacency over the tunnel interface, the interface must be included in the EIGRP process, typically with 'ip eigrp <as-number>' under the interface. Without it, EIGRP will not send or receive hello packets on that interface.
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.
✗
The DMVPN tunnel is using a different autonomous system number than the point-to-point link.
Why it's wrong here
Incorrect. If the autonomous system numbers differ, EIGRP adjacencies would not form on any interface. The scenario states adjacencies form on the point-to-point link, so AS numbers match.
✗
The tunnel interface is in a different VRF than the point-to-point link.
Why it's wrong here
Incorrect. While VRF mismatch can prevent adjacencies, the scenario does not mention VRF configuration. The most common cause is missing EIGRP configuration on the tunnel interface.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between routing protocol configuration (e.g., 'ip eigrp') and tunnel-specific parameters (e.g., NHRP), leading candidates to incorrectly blame NHRP mapping or AS number mismatches when the real issue is the missing EIGRP enable command on the interface.

Trap categories for this question

Command / output trap
Incorrect. The 'ip nhrp map' command is used for static mapping, but NHRP dynamically learns mappings in a typical DMVPN. The adjacency issue is more likely due to EIGRP not being enabled on the tunnel.
Scenario analysis trap
Incorrect. If the autonomous system numbers differ, EIGRP adjacencies would not form on any interface. The scenario states adjacencies form on the point-to-point link, so AS numbers match.

Detailed technical explanation

How to think about this question

EIGRP uses multicast Hello packets (224.0.0.10) to discover neighbors. On a DMVPN tunnel interface, the 'ip eigrp' command enables EIGRP on that interface, allowing it to send Hellos over the tunnel. Without it, the interface is effectively silent for EIGRP, and no adjacency can form. In real-world deployments, this is a common misconfiguration when adding EIGRP to a DMVPN spoke, as engineers often focus on NHRP and tunnel settings but forget to enable the routing protocol on the tunnel interface.

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: The tunnel interface is not configured with the 'ip eigrp' command under the interface configuration. — Option B is correct because the most common reason for EIGRP not forming an adjacency over a tunnel interface is the absence of the 'ip eigrp' command under that interface. Without this command, EIGRP is not enabled on the tunnel, so it will not send or process Hello packets, preventing adjacency formation even if NHRP and tunnel parameters are correctly configured.

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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