The answer is a Phase 2 encryption and authentication algorithm mismatch. This is the most likely cause because in IKEv2, Phase 2 proposals are negotiated independently from Phase 1; while Phase 1 establishes the secure channel using its own set of algorithms, Phase 2 negotiates the actual data traffic protection parameters. When the `diagnose vpn ike log` shows a "proposal mismatch" error after a successful Phase 1, it directly indicates that the Phase 2 settings—such as encryption algorithms (like AES128 vs AES256), authentication algorithms (like SHA1 vs SHA256), and Perfect Forward Secrecy (PFS) group—do not align between the two FortiGate peers. On the Fortinet NSE 4 Network Security Professional exam, this scenario tests your understanding of IKEv2’s two-phase negotiation flow, a common trap where students mistakenly blame Phase 1 settings. A useful memory tip: think of Phase 1 as the "handshake" and Phase 2 as the "data lock"—if the lock’s key doesn’t match, the handshake is useless.
NSE4 Authentication and VPN Practice Question
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 admin configures an IPsec VPN between two FortiGates using IKEv2. Phase 1 completes successfully, but Phase 2 fails to establish. The admin 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.
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 encryption and authentication algorithms do not match
In IKEv2, Phase 2 proposals are negotiated separately from Phase 1. A proposal mismatch error indicates that the Phase 2 parameters (such as encryption algorithm, authentication algorithm, and PFS settings) 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 not compatible
Why it's wrong here
Both are using IKEv2; that would be a Phase 1 issue.
✗
The Phase 2 selectors (local and remote subnets) are misconfigured
Why it's wrong here
While incorrect selectors can cause issues, the error specifically indicates a proposal mismatch, not a selector mismatch.
✓
The Phase 2 encryption and authentication algorithms do not match
Why this is correct
The 'proposal mismatch' error typically means the Phase 2 transform sets (encryption, authentication, PFS) differ between the two FortiGates.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Pre-shared keys are used in Phase 1 authentication; a mismatch would cause Phase 1 to fail.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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 encryption and authentication algorithms do not match — In IKEv2, Phase 2 proposals are negotiated separately from Phase 1. A proposal mismatch error indicates that the Phase 2 parameters (such as encryption algorithm, authentication algorithm, and PFS settings) 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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This NSE4 practice question is part of Courseiva's free Fortinet certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the NSE4 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.