This NSE4 practice question tests your understanding of authentication 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 configures an IPsec VPN between two FortiGate devices. Phase 1 completes successfully, but Phase 2 fails to establish. The administrator runs 'diagnose vpn ike log' and sees the error 'proposal mismatch'. 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.
Why wrong: IKE version mismatch causes Phase 1 failure, not a Phase 2 proposal mismatch.
B
The pre-shared key is incorrect
Why wrong: An incorrect PSK would cause Phase 1 to fail, not Phase 2.
C
The firewall policies are blocking IKE traffic on UDP port 500
Why wrong: Blocking IKE traffic would prevent Phase 1 from completing, not cause a Phase 2 proposal mismatch.
D
The Phase 2 local and remote subnets do not match on both ends
Phase 2 proposal mismatch typically occurs when the subnets defined in the Phase 2 selectors or the encryption/authentication parameters do not match between the peers.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Phase 2 local and remote subnets do not match on both ends
Option C is correct. A proposal mismatch in Phase 2 indicates that the Phase 2 selectors (local/remote subnets, protocol, port) or the SA proposal parameters (encryption, authentication, PFS) do not match between the two peers.
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 IKE version is mismatched (IKEv1 vs IKEv2)
Why it's wrong here
IKE version mismatch causes Phase 1 failure, not a Phase 2 proposal mismatch.
✗
The pre-shared key is incorrect
Why it's wrong here
An incorrect PSK would cause Phase 1 to fail, not Phase 2.
✗
The firewall policies are blocking IKE traffic on UDP port 500
Why it's wrong here
Blocking IKE traffic would prevent Phase 1 from completing, not cause a Phase 2 proposal mismatch.
✓
The Phase 2 local and remote subnets do not match on both ends
Why this is correct
Phase 2 proposal mismatch typically occurs when the subnets defined in the Phase 2 selectors or the encryption/authentication parameters do not match between the peers.
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 NSE4 subnetting questions on CIDR, address ranges, and subnet selection.
Authentication and VPN — This question tests Authentication and VPN — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The Phase 2 local and remote subnets do not match on both ends — Option C is correct. A proposal mismatch in Phase 2 indicates that the Phase 2 selectors (local/remote subnets, protocol, port) or the SA proposal parameters (encryption, authentication, PFS) do not match between the two peers.
What should I do if I get this NSE4 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 NSE4 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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