The answer is that a Phase 2 proposal mismatch in IPsec VPN is most commonly caused by mismatched encryption or authentication algorithms and mismatched proxy IDs between the two peers. This occurs because Phase 2 negotiations use a proposal exchange where each peer sends its configured set of parameters, including encryption algorithms like AES128 or AES256, authentication hashes, and the specific local and remote subnets defined as proxy IDs; if any of these values do not align exactly with what the remote peer expects, the negotiation fails with the 'no matching proposal' error. On the Fortinet NSE 7 Advanced Security NSE7 exam, this question tests your ability to distinguish between Phase 2 configuration errors and unrelated issues like firewall policies or routing, which are common traps—remember that Phase 2 is about data-plane protection, not connectivity. A useful memory tip is to think of Phase 2 as a "double match": you need both the crypto suite and the traffic selectors (proxy IDs) to match, or the tunnel will not establish.
NSE7 Troubleshooting and Diagnostics Practice Question
This NSE7 practice question tests your understanding of troubleshooting and diagnostics. 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 Phase 2 negotiation failure. The debug shows 'no matching phase 2 proposal' from the remote peer. Which TWO of the following are likely causes? (Choose two.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The local and remote proxy IDs (subnets) are not matching
Options B and D are correct. Phase 2 proposal mismatch is usually due to incompatible encryption/authentication algorithms (B) or mismatched proxy IDs (local/remote subnets) (D). Options A and C would not cause a proposal mismatch.
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 local and remote proxy IDs (subnets) are not matching
Why this is correct
Phase 2 requires matching proxy IDs to establish SAs.
IKE version mismatch prevents Phase 1 from completing, not 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 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 NSE7 subnetting questions on CIDR, address ranges, and subnet selection.
Troubleshooting and Diagnostics — This question tests Troubleshooting and Diagnostics — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The local and remote proxy IDs (subnets) are not matching — Options B and D are correct. Phase 2 proposal mismatch is usually due to incompatible encryption/authentication algorithms (B) or mismatched proxy IDs (local/remote subnets) (D). Options A and C would not cause a proposal mismatch.
What should I do if I get this NSE7 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 NSE7 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
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These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A site-to-site IPsec VPN tunnel is failing. The administrator runs 'diagnose vpn ike config' and sees that phase 1 parameters are correct. However, phase 2 negotiation fails with 'no proposal chosen'. What is the MOST likely cause?
medium
A.The pre-shared key is incorrect
✓ B.The phase 2 encryption/authentication algorithms do not match between peers
C.The firewall policy allowing IKE traffic is missing
D.The remote gateway IP address is wrong
Why B: Option A is correct. The 'no proposal chosen' error in phase 2 indicates a mismatch in the phase 2 parameters (encryption, authentication, PFS, etc.) between the two peers. The administrator should verify that the phase 2 selectors and proposals match on both ends.
Variation 2. A network administrator is troubleshooting an IPsec VPN tunnel that fails to establish. The remote gateway logs show a proposal mismatch. On FortiGate, the administrator runs 'diagnose vpn ike config' and sees 'proposal: aes128-sha1, aes256-sha256'. The remote side expects 'aes256-sha1'. What is the most likely cause?
medium
✓ A.The Phase 1 proposal list does not include the algorithm combination the remote gateway requires
B.The pre-shared key is incorrect
C.The Phase 2 selectors are misconfigured
D.The IKE version is set to 1 but remote uses 2
Why A: The proposal mismatch occurs because the FortiGate's IKE proposal includes aes256-sha256 but not aes256-sha1, and the remote gateway expects aes256-sha1. The correct action is to add aes256-sha1 to the proposal list.
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This NSE7 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 NSE7 exam.
Question Discussion
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