The correct answer is that the loopback0 interface is not included in the OSPF process. For a VXLAN tunnel to become operational, the NVE source interface—typically a loopback—must be reachable by other VTEPs via the underlay routing protocol. Even though an OSPF adjacency is established, if the loopback0 interface is not explicitly advertised under the OSPF process, its IP address remains unreachable, preventing the VXLAN tunnel from coming up. This scenario directly tests your understanding of VXLAN underlay OSPF source interface configuration, a common pitfall on the Cisco DCCOR 350-601 exam. The trap is that engineers often verify OSPF neighbor states but forget to verify that the source interface itself is part of the OSPF network statement. A helpful memory tip: “If the source isn’t sourced, the tunnel is divorced”—always confirm your loopback is injected into the underlay routing process.
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.
Exhibit
interface nve1
source-interface loopback0
member vni 10000 mcast-group 239.1.1.1
!
interface loopback0
ip address 10.10.10.1 255.255.255.255
!
interface Ethernet1/1
no switchport
ip address 192.168.1.1 255.255.255.0
ip router ospf 1 area 0
Refer to the exhibit. An engineer configured a VXLAN tunnel endpoint (VTEP) but the VXLAN tunnel is not operational. The underlay OSPF adjacency is established. What is the missing configuration?
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 included in the OSPF process.
Option B is correct because the loopback0 interface is the source interface for the NVE, but it is not included in the OSPF process, making its IP unreachable from other VTEPs. Option A is incorrect because NVE interfaces are typically administratively up by default. Option C is incorrect because multicast reachability is not directly related to the tunnel operational status. Option D is incorrect because VNI-to-VLAN mapping is not shown in the exhibit and is required for Layer 2 forwarding, but the immediate issue is the unreachable source interface.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 NVE interface must be enabled with the no shutdown command.
Why it's wrong here
NVE interfaces are usually administratively up by default; no shutdown is not typically required.
✗
The multicast group must be reachable via the underlay.
Why it's wrong here
Multicast reachability affects BUM traffic but not the tunnel's operational status.
✓
The loopback0 interface is not included in the OSPF process.
Why this is correct
The loopback0 interface, used as the NVE source, is not advertised via OSPF, so its IP is unreachable from other VTEPs.
VNI-to-VLAN mapping is needed for Layer 2 forwarding, but the immediate issue is the unreachable source interface.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-601 OSPF questions on adjacency and route selection.
Network — This question tests Network — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The loopback0 interface is not included in the OSPF process. — Option B is correct because the loopback0 interface is the source interface for the NVE, but it is not included in the OSPF process, making its IP unreachable from other VTEPs. Option A is incorrect because NVE interfaces are typically administratively up by default. Option C is incorrect because multicast reachability is not directly related to the tunnel operational status. Option D is incorrect because VNI-to-VLAN mapping is not shown in the exhibit and is required for Layer 2 forwarding, but the immediate issue is the unreachable source interface.
What should I do if I get this 350-601 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-601 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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