- A
The proxy IDs (local/remote subnets) do not match between peers
Mismatched proxy IDs are the most common cause of phase 2 failure.
- B
The tunnel is configured as route-based instead of policy-based
Why wrong: Route-based VPNs also require proxy IDs for phase 2.
- C
The IKE gateway's local interface is down
Why wrong: Phase 1 would not complete if the interface were down.
- D
Dead peer detection is not enabled on the IKE gateway
Why wrong: DPD does not affect IPsec proposal negotiation.
Question 191 of 516
IPSec Phase 2 Failure: Proxy ID Mismatch
This PCNSE practice question tests your understanding of secure access 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 is troubleshooting an IPsec site-to-site VPN that fails to establish. IKE phase 1 completes successfully, but phase 2 fails with a 'no proposal chosen' message. Both sides have identical IKE and IPsec crypto profiles, and the pre-shared key is correct. What is the most likely cause of the failure?
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 choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The proxy IDs (local/remote subnets) do not match between peers
In IPsec site-to-site VPNs, IKE phase 1 establishes the secure management channel using parameters like encryption, authentication, and Diffie-Hellman groups. Phase 2 negotiates the IPsec security associations (SAs) for actual data traffic, and the 'no proposal chosen' error indicates a mismatch in the phase 2 parameters. Since both sides have identical crypto profiles and the pre-shared key is correct, the most likely cause is that the proxy IDs (local and remote subnets) do not match between peers. Proxy IDs define the traffic selectors that each peer expects to protect; if they are misaligned, the IPsec SA negotiation fails even if all other settings are identical.
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 proxy IDs (local/remote subnets) do not match between peers
Why this is correct
Mismatched proxy IDs are the most common cause of phase 2 failure.
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.
- ✗
The tunnel is configured as route-based instead of policy-based
Why it's wrong here
Route-based VPNs also require proxy IDs for phase 2.
- ✗
The IKE gateway's local interface is down
Why it's wrong here
Phase 1 would not complete if the interface were down.
- ✗
Dead peer detection is not enabled on the IKE gateway
Why it's wrong here
DPD does not affect IPsec proposal negotiation.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume identical crypto profiles guarantee phase 2 success, overlooking that proxy IDs (traffic selectors) are a separate, critical parameter that must be mirrored exactly on both peers.
Detailed technical explanation
How to think about this question
Proxy IDs in Palo Alto Networks firewalls correspond to the local and remote subnets defined in the IPsec crypto profile, which are used as traffic selectors during IKEv1 phase 2 (Quick Mode) or IKEv2 CREATE_CHILD_SA exchanges. If the proxy IDs are mismatched—for example, one peer specifies 10.1.1.0/24 to 10.2.2.0/24 while the other specifies 10.1.1.0/24 to 10.3.3.0/24—the responder will reject the proposal with a 'no proposal chosen' notification. This is a common misconfiguration when administrators copy configurations without verifying subnet definitions, especially in hub-and-spoke topologies.
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.
Visual reference
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
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.
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FAQ
Questions learners often ask
What does this PCNSE question test?
Secure Access and VPN — This question tests Secure Access and VPN — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The proxy IDs (local/remote subnets) do not match between peers — In IPsec site-to-site VPNs, IKE phase 1 establishes the secure management channel using parameters like encryption, authentication, and Diffie-Hellman groups. Phase 2 negotiates the IPsec security associations (SAs) for actual data traffic, and the 'no proposal chosen' error indicates a mismatch in the phase 2 parameters. Since both sides have identical crypto profiles and the pre-shared key is correct, the most likely cause is that the proxy IDs (local and remote subnets) do not match between peers. Proxy IDs define the traffic selectors that each peer expects to protect; if they are misaligned, the IPsec SA negotiation fails even if all other settings are identical.
What should I do if I get this PCNSE 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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Last reviewed: Jul 4, 2026
This PCNSE practice question is part of Courseiva's free Palo Alto Networks 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 PCNSE exam.
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