This PCNSE practice question tests your understanding of secure access and vpn. 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 network administrator is troubleshooting an IPsec site-to-site VPN that fails to establish. IKE phase 1 completes successfully, but phase 2 fails with a 'no proposal chosen' message. Both sides have identical IKE and IPsec crypto profiles, and the pre-shared key is correct. What is the most likely cause of the failure?
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 proxy IDs (local/remote subnets) do not match between peers
Option C is correct because phase 2 failure with matching crypto profiles typically indicates a proxy ID mismatch. Option A is incorrect because route-based VPNs still require proxy IDs. Option B is incorrect because the IKE gateway is active. Option D is incorrect because dead peer detection settings do not affect phase 2 proposal negotiation.
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 proxy IDs (local/remote subnets) do not match between peers
Why this is correct
Mismatched proxy IDs are the most common cause of phase 2 failure.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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 PCNSE subnetting questions on CIDR, address ranges, and subnet selection.
Secure Access and VPN — This question tests Secure Access and VPN — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The proxy IDs (local/remote subnets) do not match between peers — Option C is correct because phase 2 failure with matching crypto profiles typically indicates a proxy ID mismatch. Option A is incorrect because route-based VPNs still require proxy IDs. Option B is incorrect because the IKE gateway is active. Option D is incorrect because dead peer detection settings do not affect phase 2 proposal negotiation.
What should I do if I get this PCNSE 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 PCNSE 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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Question Discussion
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