Question 361 of 500

Network →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is the loopback0 interface is not reachable via the underlay network. This is the most likely cause because the NVE interface depends entirely on IP reachability to its specified source interface across the underlay to form VXLAN tunnels; if OSPF or IS-IS routes are missing or the underlay configuration is incorrect, the NVE interface will remain down even when all physical interfaces are up. On the Cisco DCCOR / CCNP Data Center Core 350-601 exam, this question tests your understanding that the NVE interface is a logical construct that requires a functional underlay routing path—a common trap is to focus on physical link status while overlooking the routing table. Remember the memory tip: “NVE needs a route to its source; no route, no tunnel.”

350-601 Network Practice Question

This 350-601 practice question tests your understanding of network. 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.

A network engineer is configuring Cisco Nexus VXLAN with BGP EVPN. The VTEPs are using loopback0 as the NVE source. The physical interfaces are up, but the NVE interface remains down. 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 1hardmultiple choice

Open the full BGP breakdown →

A
The loopback0 interface is not reachable via the underlay network.
The NVE source must be routable in the underlay. If loopback0 is not advertised by IGP, the NVE interface stays down.
B
The NVE interface is not configured with source-interface.
Why wrong: If source-interface were not configured, the NVE interface would not come up. But the scenario states the configuration is in place.
C
The VLAN 1 is not associated with the NVE interface.
Why wrong: VLAN association is for VNI mapping, not for NVE interface state. The NVE interface state depends on source-interface reachability.
D
The loopback0 interface is not created.
Why wrong: If loopback0 were not created, the switch would not accept 'source-interface loopback0' in the NVE configuration; it would be a config error, not just NVE down.

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 loopback0 interface is not reachable via the underlay network.

The NVE interface requires the specified source interface (loopback0) to have IP reachability via the underlay network to establish VXLAN tunnels. If loopback0 is not reachable (e.g., due to missing OSPF/IS-IS routes or incorrect underlay configuration), the NVE interface will remain in a down state even if the physical interfaces are up. This is because the NVE interface depends on the underlay routing to encapsulate and forward VXLAN traffic.

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 loopback0 interface is not reachable via the underlay network.
Why this is correct
The NVE source must be routable in the underlay. If loopback0 is not advertised by IGP, the NVE interface stays down.
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 NVE interface is not configured with source-interface.
Why it's wrong here
If source-interface were not configured, the NVE interface would not come up. But the scenario states the configuration is in place.
✗
The VLAN 1 is not associated with the NVE interface.
Why it's wrong here
VLAN association is for VNI mapping, not for NVE interface state. The NVE interface state depends on source-interface reachability.
✗
The loopback0 interface is not created.
Why it's wrong here
If loopback0 were not created, the switch would not accept 'source-interface loopback0' in the NVE configuration; it would be a config error, not just NVE down.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the dependency of the NVE interface on underlay IP reachability, tricking candidates into focusing on NVE-specific configuration errors (like missing source-interface) rather than verifying the underlay routing for the loopback address.

Trap categories for this question

Scenario analysis trap
If source-interface were not configured, the NVE interface would not come up. But the scenario states the configuration is in place.

Detailed technical explanation

How to think about this question

Under the hood, the NVE interface uses the source IP (loopback0) as the tunnel endpoint for VXLAN. The BGP EVPN control plane relies on this IP to exchange Type-2 and Type-3 routes; if the underlay (e.g., OSPF or IS-IS) does not advertise loopback0, the VTEP cannot respond to ARP or NDP for that IP, causing the NVE to remain in a down/down state. In real-world scenarios, this often occurs when the underlay routing protocol is misconfigured or when the loopback interface is not included in the network statement.

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 350-601 question test?

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

What is the correct answer to this question?

The correct answer is: The loopback0 interface is not reachable via the underlay network. — The NVE interface requires the specified source interface (loopback0) to have IP reachability via the underlay network to establish VXLAN tunnels. If loopback0 is not reachable (e.g., due to missing OSPF/IS-IS routes or incorrect underlay configuration), the NVE interface will remain in a down state even if the physical interfaces are up. This is because the NVE interface depends on the underlay routing to encapsulate and forward VXLAN traffic.

What should I do if I get this 350-601 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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