The answer is that the loopback0 is not advertised in the routing protocol. In a FlexVPN hub-and-spoke topology, the tunnel interface establishes the encrypted overlay, but reachability to the hub’s loopback address depends on that subnet being explicitly injected into the dynamic routing protocol—such as EIGRP or OSPF—via a network statement or redistribution. Without this, spokes learn routes only for the tunnel network and any spoke subnets, leaving the loopback unreachable even though IKEv2 and the tunnel itself are fully operational. On the ENCOR 350-401 exam, this scenario tests your understanding that FlexVPN routing is separate from tunnel establishment; a common trap is to assume a working tunnel guarantees all hub interfaces are reachable. Remember the memory tip: “Tunnel up does not mean loopback learned”—always verify the routing protocol’s network statement includes the loopback.
350-401 VPN Technologies Practice Question
This 350-401 practice question tests your understanding of vpn technologies. 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.
An engineer is configuring a FlexVPN hub-and-spoke network. The hub router has a loopback0 with IP 10.0.0.1/32. The spokes are configured to use IKEv2 with certificates. The engineer notices that the spokes can establish the IKEv2 tunnel and can ping the hub's tunnel IP, but cannot reach the loopback0 address. The hub has a static route for the spoke subnets. What is the most likely issue?
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 loopback0 is not advertised in the routing protocol.
In FlexVPN, the tunnel IP addresses are typically used for routing, and the loopback may not be advertised into the routing protocol or may not be reachable via the tunnel interface. If the hub's loopback is not included in the routing updates (e.g., via a network statement in EIGRP or OSPF), the spokes will not have a route to it. Option D is correct because the loopback is not being advertised. Option A is incorrect because IKEv2 is working. Option B is incorrect because certificates are not the issue. Option C is incorrect because the tunnel itself is up.
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 IKEv2 proposal does not match between hub and spoke.
Why it's wrong here
The tunnel is established, so IKEv2 is working.
✗
The certificate authority is not trusted by the hub.
Why it's wrong here
Certificate trust issues would prevent tunnel establishment.
✗
The tunnel interface is not in an up/up state.
Why it's wrong here
The tunnel is up because the spokes can ping the tunnel IP.
✓
The loopback0 is not advertised in the routing protocol.
Why this is correct
Correct because without a route, the spokes cannot reach the loopback.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
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 350-401 OSPF questions on adjacency and route selection.
VPN Technologies — This question tests VPN Technologies — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The loopback0 is not advertised in the routing protocol. — In FlexVPN, the tunnel IP addresses are typically used for routing, and the loopback may not be advertised into the routing protocol or may not be reachable via the tunnel interface. If the hub's loopback is not included in the routing updates (e.g., via a network statement in EIGRP or OSPF), the spokes will not have a route to it. Option D is correct because the loopback is not being advertised. Option A is incorrect because IKEv2 is working. Option B is incorrect because certificates are not the issue. Option C is incorrect because the tunnel itself is up.
What should I do if I get this 350-401 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 350-401 OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
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