The answer is a mismatched Phase 2 proxy ID, specifically the local and remote subnet definitions. This is the most likely cause because when Phase 1 successfully completes but Phase 2 fails with "no proposal chosen," the IKE negotiation has already established the secure channel for authentication, yet the IPsec security association cannot agree on which traffic to protect. The Phase 2 proposal includes the proxy IDs, which define the source and destination subnets for encryption; if the local FortiGate’s proxy IDs do not exactly match one of the multiple Phase 2 selectors configured on the remote FortiGate, the negotiation fails. On the Fortinet NSE 7 Advanced Security exam, this scenario tests your understanding of IPsec SA parameters and the critical role of proxy IDs in multi-selector environments—a common trap is assuming Phase 1 success guarantees Phase 2 success, or misreading the logs as a proposal mismatch in encryption algorithms. Remember the mnemonic: “Phase 1 is the handshake, Phase 2 is the map—if the proxy IDs don’t match, the traffic won’t wrap.”
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 a scenario where IPSec VPN tunnels between two FortiGates are flapping. The logs show Phase 1 is up but Phase 2 fails with 'no proposal chosen'. The remote FortiGate has multiple Phase 2 selectors configured. 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.
The 'no proposal chosen' error during Phase 2, despite Phase 1 being up, indicates a mismatch in the IPsec security association (SA) parameters. Since the remote FortiGate has multiple Phase 2 selectors configured, the most likely cause is that the local and remote proxy IDs (local and remote subnets) do not match any of the configured selectors. Phase 2 negotiation uses these proxy IDs to define which traffic should be encrypted; if they don't align, the IKE SA cannot be established.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 error 'no proposal chosen' is often due to mismatched proxy IDs in Phase 2.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Mismatched pre-shared keys.
Why it's wrong here
Pre-shared key mismatch would cause Phase 1 to fail, not Phase 2.
✗
Dead Peer Detection (DPD) settings are too aggressive.
Why it's wrong here
DPD issues would cause the tunnel to drop after being established, not a Phase 2 failure.
✗
Certificate validation failure.
Why it's wrong here
Certificate issues would affect Phase 1 if certificate authentication is used, but logs show Phase 1 is up.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse Phase 1 and Phase 2 failures, assuming any 'no proposal chosen' error is due to Phase 1 misconfigurations like PSK or certificates, when in fact Phase 1 is already up, isolating the issue to Phase 2 proxy ID mismatches.
Trap categories for this question
Command / output trap
Certificate issues would affect Phase 1 if certificate authentication is used, but logs show Phase 1 is up.
Detailed technical explanation
How to think about this question
In IPsec, Phase 2 (Quick Mode) negotiates the IPsec SA using proxy IDs, which consist of local/remote subnets and protocol/port. FortiGate uses the 'set src-addr' and 'dst-addr' in the Phase 2 configuration to match these. If the remote FortiGate has multiple Phase 2 selectors (e.g., different subnets), the local FortiGate's proxy IDs must exactly match one of them; otherwise, the IKE daemon will reject the proposal with a 'no proposal chosen' notification (RFC 2409, Section 5.5). A real-world scenario is when an administrator adds a new subnet to the remote selector but forgets to update the local side, causing intermittent failures for specific traffic flows.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Troubleshooting and Diagnostics — This question tests Troubleshooting and Diagnostics — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Mismatched Phase 2 proxy IDs (local/remote subnets). — The 'no proposal chosen' error during Phase 2, despite Phase 1 being up, indicates a mismatch in the IPsec security association (SA) parameters. Since the remote FortiGate has multiple Phase 2 selectors configured, the most likely cause is that the local and remote proxy IDs (local and remote subnets) do not match any of the configured selectors. Phase 2 negotiation uses these proxy IDs to define which traffic should be encrypted; if they don't align, the IKE SA cannot be established.
What should I do if I get this NSE7 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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Question Discussion
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