The answer is that the new loopback IP 10.1.1.105 is not included in the OSPF network statement under router ospf. This is because after a loopback IP change, the OSPF network statement must explicitly cover the new address for the route to be advertised into the underlay; without it, the OSPF database will not contain the new VTEP IP, and other leaves cannot route to it, causing VXLAN tunnels to fail. On the Cisco DCCOR / CCNP Data Center Core 350-601 exam, this scenario tests your understanding that OSPF adjacencies remain FULL over physical interfaces even when a loopback IP is missing from the network statement—a common trap where engineers mistakenly check adjacency status instead of route reachability. The key memory tip is “VTEP IPs must be in the OSPF network statement, not just the physical interface IPs,” or simply remember: if tunnels drop but adjacencies are up, check the loopback’s OSPF advertisement first.
350-601 Network Practice Question
This 350-601 practice question tests your understanding of network. 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 large enterprise uses Cisco Nexus 9000 switches in a VXLAN EVPN fabric. The underlay is OSPF. Each leaf switch has a loopback0 interface as the source interface for VXLAN tunnel endpoints. After a maintenance window, an engineer modifies the IP address of loopback0 on leaf-5 from 10.1.1.5/32 to 10.1.1.105/32. Subsequently, all VXLAN tunnels to leaf-5 go down. OSPF adjacencies between leaf-5 and the spines are still FULL. The engineer checks the NVE interface on leaf-5 and sees the source-interface is loopback0 but the interface status is up/up. However, pings from other leaves to 10.1.1.105 fail. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The new loopback IP 10.1.1.105 is not included in the OSPF network statement under router ospf
The correct answer is B. After changing the loopback0 IP address on leaf-5, the new IP 10.1.1.105/32 must be explicitly advertised into OSPF for other leaves to reach it. If the OSPF network statement under router ospf still references the old subnet or does not include 10.1.1.105/32, the route for this new loopback IP will not be installed in the OSPF database. Consequently, other leaves cannot route to the new VTEP IP, causing VXLAN tunnels to fail even though OSPF adjacencies remain FULL (since adjacencies are formed over physical interfaces, not the loopback).
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 OSPF process on leaf-5 was not restarted after the IP change
Why it's wrong here
OSPF dynamically adjusts to IP changes; restart is not required.
✓
The new loopback IP 10.1.1.105 is not included in the OSPF network statement under router ospf
Why this is correct
The new IP subnet must be advertised via OSPF to be reachable by other leaves.
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 MTU on loopback0 is set too low causing OSPF hello drops
Why it's wrong here
OSPF adjacencies are still FULL, so MTU is not an issue.
✗
The VXLAN source-interface was automatically changed to a different loopback
Why it's wrong here
The source-interface remains configured as loopback0; it does not change automatically.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between OSPF adjacency status (which relies on physical interfaces) and route advertisement (which depends on network statements covering the loopback IP), leading candidates to incorrectly assume that FULL adjacencies guarantee reachability to the VTEP IP.
Detailed technical explanation
How to think about this question
In OSPF, the network statement uses a wildcard mask to match interface IPs; if the new loopback IP falls outside the range of any configured network statement, OSPF will not advertise that IP as a stub host route. This is a common misconfiguration when migrating VTEP IPs in a VXLAN EVPN fabric, as the underlay routing must explicitly carry the new /32 host route for BGP EVPN peering and VXLAN tunnel establishment. The OSPF adjacency remains FULL because it is maintained on the physical underlay links, not on the loopback 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.
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 new loopback IP 10.1.1.105 is not included in the OSPF network statement under router ospf — The correct answer is B. After changing the loopback0 IP address on leaf-5, the new IP 10.1.1.105/32 must be explicitly advertised into OSPF for other leaves to reach it. If the OSPF network statement under router ospf still references the old subnet or does not include 10.1.1.105/32, the route for this new loopback IP will not be installed in the OSPF database. Consequently, other leaves cannot route to the new VTEP IP, causing VXLAN tunnels to fail even though OSPF adjacencies remain FULL (since adjacencies are formed over physical interfaces, not the loopback).
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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