- A
The spoke is configured with a static IP address on the tunnel interface that is not in the hub's IP pool.
Correct. In FlexVPN, the hub assigns IP addresses from a pool. If the spoke statically configures an IP address, the hub may not have a route back to that address, causing asymmetric routing or unreachability.
- B
The hub is missing the 'tunnel protection ipsec' command on the tunnel interface.
Why wrong: Incorrect. If tunnel protection were missing, the tunnel would not come up at all. The tunnel is up, so protection is configured.
- C
The spoke's crypto map is not using the correct pre-shared key.
Why wrong: Incorrect. If the pre-shared key were incorrect, Phase 1 would fail, and the tunnel would not come up.
- D
The hub's IKEv2 profile is not configured with 'authentication remote rsa-sig'.
Why wrong: Incorrect. The authentication method must match between peers. If it didn't match, the tunnel would not establish. The tunnel is up, so authentication is successful.
Quick Answer
The answer is a static IP address on the spoke’s tunnel interface that falls outside the hub’s assigned IP pool. In a FlexVPN hub-and-spoke topology using IKEv2, the hub uses a local pool to dynamically assign tunnel IP addresses to spokes, and the spoke’s tunnel interface must be configured to receive its address via DHCP or from the hub’s pool. When the spoke statically sets an IP not in that pool, the hub lacks a proper reverse route to the spoke’s tunnel network, breaking return traffic—hence the spoke cannot ping the hub’s tunnel IP, even though the hub can ping the spoke because it sees the spoke’s address as directly connected. On the ENCOR 350-401 exam, this tests your understanding of FlexVPN address assignment and route injection; a common trap is assuming any static IP works as long as the tunnel is up. Memory tip: “If the spoke’s tunnel IP isn’t in the hub’s pool, the ping’s a fool.”
350-401 WAN Technologies Practice Question
This 350-401 practice question tests your understanding of wan 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 topology using IKEv2. The hub router is configured with a dynamic crypto map and a local pool for assigning IP addresses to spokes. The spokes are configured with a static crypto map and a tunnel interface with an IP address from the pool. The tunnel comes up, but the spoke cannot ping the hub's tunnel interface. The hub can ping the spoke's tunnel interface. 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 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 spoke is configured with a static IP address on the tunnel interface that is not in the hub's IP pool.
In FlexVPN, the hub assigns an IP address to the spoke from a pool. The spoke's tunnel interface should receive this IP address dynamically. If the spoke is configured with a static IP address that is not in the hub's pool, the hub will not route traffic back to the spoke correctly, or the spoke may have a mismatched subnet. The hub can ping the spoke because the spoke's tunnel IP is reachable, but the spoke cannot ping the hub because the spoke's routing table may not have a route to the hub's tunnel IP, or the hub's reverse route injection is not working.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 spoke is configured with a static IP address on the tunnel interface that is not in the hub's IP pool.
Why this is correct
Correct. In FlexVPN, the hub assigns IP addresses from a pool. If the spoke statically configures an IP address, the hub may not have a route back to that address, causing asymmetric routing or unreachability.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The hub is missing the 'tunnel protection ipsec' command on the tunnel interface.
Why it's wrong here
Incorrect. If tunnel protection were missing, the tunnel would not come up at all. The tunnel is up, so protection is configured.
- ✗
The spoke's crypto map is not using the correct pre-shared key.
Why it's wrong here
Incorrect. If the pre-shared key were incorrect, Phase 1 would fail, and the tunnel would not come up.
- ✗
The hub's IKEv2 profile is not configured with 'authentication remote rsa-sig'.
Why it's wrong here
Incorrect. The authentication method must match between peers. If it didn't match, the tunnel would not establish. The tunnel is up, so authentication is successful.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 350-401 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this 350-401 question test?
WAN Technologies — This question tests WAN Technologies — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The spoke is configured with a static IP address on the tunnel interface that is not in the hub's IP pool. — In FlexVPN, the hub assigns an IP address to the spoke from a pool. The spoke's tunnel interface should receive this IP address dynamically. If the spoke is configured with a static IP address that is not in the hub's pool, the hub will not route traffic back to the spoke correctly, or the spoke may have a mismatched subnet. The hub can ping the spoke because the spoke's tunnel IP is reachable, but the spoke cannot ping the hub because the spoke's routing table may not have a route to the hub's tunnel IP, or the hub's reverse route injection is not working.
What should I do if I get this 350-401 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 350-401 subnetting questions on CIDR, address ranges, and subnet 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?
CIDR notation defines the prefix length.
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Last reviewed: Jun 18, 2026
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