350-601 Network Practice Question
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?
⚠ Common exam trap
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.
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 new loopback IP 10.1.1.105 is not included in the OSPF network statement under router ospf
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).
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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