The answer is the Phase 2 encryption algorithm and the PFS (Perfect Forward Secrecy) settings. When an IPsec VPN Phase 2 shows “no matching proposal” despite Phase 1 being up, it means the two FortiGates cannot agree on the parameters for the data tunnel, and the most common mismatches are the encryption algorithm (e.g., AES128 vs. AES256) and the PFS group (e.g., Diffie-Hellman group 14 vs. group 2). On the Fortinet NSE 4 exam, this scenario tests your understanding that Phase 2 proposals must be an exact mirror on both ends—unlike Phase 1, which allows a proposal list. A common trap is assuming the error is always a subnet mismatch, but the “no matching proposal” log entry specifically points to cryptographic parameter disagreement. Remember the mnemonic “P2 E-PFS”: Encryption and PFS must match first, then check selectors.
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.
An administrator is troubleshooting an IPsec VPN between two FortiGates. Phase 1 is up but Phase 2 is down. The admin runs 'diagnose vpn ike log' and sees 'no matching proposal'. To resolve this issue, which TWO settings should be checked on both ends?
PFS group must match; if one peer has PFS disabled and the other has it enabled, Phase 2 will fail.
B
Phase 1 authentication method
Why wrong: Already negotiated successfully in Phase 1.
C
Phase 2 local and remote subnets
Why wrong: While mismatched subnets can cause issues, the error 'no matching proposal' typically refers to algorithms/PFS, not selectors. However, sometimes selectors are included in the proposal; but the two most common causes are encryption and PFS.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Phase 2 PFS (Perfect Forward Secrecy) group
Phase 2 parameters must match exactly, especially encryption algorithm and PFS settings. The proposal mismatch can also be caused by mismatched selectors (subnets), but the question asks for the two settings most directly related to the 'no matching proposal' error.
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.
✓
Phase 2 PFS (Perfect Forward Secrecy) group
Why this is correct
PFS group must match; if one peer has PFS disabled and the other has it enabled, Phase 2 will fail.
While mismatched subnets can cause issues, the error 'no matching proposal' typically refers to algorithms/PFS, not selectors. However, sometimes selectors are included in the proposal; but the two most common causes are encryption and PFS.
Phase 1 is already up, so its settings are fine; the error is in Phase 2.
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 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: Phase 2 PFS (Perfect Forward Secrecy) group — Phase 2 parameters must match exactly, especially encryption algorithm and PFS settings. The proposal mismatch can also be caused by mismatched selectors (subnets), but the question asks for the two settings most directly related to the 'no matching proposal' error.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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