- A
The IKE phase 1 proposal is mismatched.
Why wrong: Incorrect. Phase 1 mismatch would prevent IKE SA establishment.
- B
The proxy IDs (interesting traffic) are not configured correctly.
Correct. Mismatched proxy IDs cause the firewall to not encrypt traffic.
- C
The IPSec crypto profile uses AES-256 and the peer uses 3DES.
Why wrong: Incorrect. Crypto mismatch would prevent the tunnel from establishing.
- D
The tunnel interface MTU is set too low.
Why wrong: Incorrect. MTU issues cause fragmentation, not complete failure of all traffic.
Question 300 of 516
IPsec Proxy ID Mismatch: Tunnel Up but No Traffic
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.
An organization has two sites connected via IPSec VPN. The tunnel is up, but ICMP traffic between sites fails. No other traffic works. The firewall policy allows any-any. What is the most likely issue?
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 (interesting traffic) are not configured correctly.
When the IPSec tunnel is up but no traffic passes, the most common cause is misconfigured proxy IDs (also called interesting traffic selectors). Proxy IDs define which source/destination subnets are permitted through the tunnel; if they don't match on both peers, the tunnel may establish (IKE and IPsec SAs are created) but the firewall will not encrypt or forward traffic because it does not match the defined selectors. Since the firewall policy allows any-any, the issue is not a policy block, pointing directly to proxy ID mismatch.
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 IKE phase 1 proposal is mismatched.
Why it's wrong here
Incorrect. Phase 1 mismatch would prevent IKE SA establishment.
- ✓
The proxy IDs (interesting traffic) are not configured correctly.
- ✗
The IPSec crypto profile uses AES-256 and the peer uses 3DES.
Why it's wrong here
Incorrect. Crypto mismatch would prevent the tunnel from establishing.
- ✗
The tunnel interface MTU is set too low.
Why it's wrong here
Incorrect. MTU issues cause fragmentation, not complete failure of all traffic.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume a tunnel being 'up' means all traffic should work, but in Palo Alto Networks, the tunnel state only reflects IKE and IPsec SA establishment, not the correctness of proxy IDs which control traffic selection.
Detailed technical explanation
How to think about this question
In Palo Alto Networks firewalls, proxy IDs correspond to the IPsec security association (SA) selectors defined in RFC 4301. If the proxy IDs on one firewall specify 10.1.1.0/24 to 10.2.2.0/24 and the peer specifies 10.1.1.0/24 to 10.3.3.0/24, the tunnel will establish (because IKEv2 or IKEv1 with aggressive mode can still create SAs) but traffic to 10.2.2.0/24 will not be encrypted because the peer’s SA does not include that subnet. A real-world scenario is when using route-based VPNs with static routes—misconfigured proxy IDs can silently drop all traffic while the tunnel remains up.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
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.
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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 (interesting traffic) are not configured correctly. — When the IPSec tunnel is up but no traffic passes, the most common cause is misconfigured proxy IDs (also called interesting traffic selectors). Proxy IDs define which source/destination subnets are permitted through the tunnel; if they don't match on both peers, the tunnel may establish (IKE and IPsec SAs are created) but the firewall will not encrypt or forward traffic because it does not match the defined selectors. Since the firewall policy allows any-any, the issue is not a policy block, pointing directly to proxy ID mismatch.
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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