- A
The pre-shared key configured on one router does not match the key on the peer.
Correct. PSK mismatch causes IKE authentication to fail, preventing the tunnel from coming up.
- B
An ACL on the router's outside interface blocks UDP port 500.
Correct. UDP port 500 is required for IKE initial exchange; blocking it prevents IKE from starting.
- C
An ACL on the router's outside interface blocks UDP port 4500.
Why wrong: Incorrect. Blocking UDP port 4500 only affects NAT-T; if NAT is not present, IKE still works on port 500.
- D
The IPsec SA lifetime is set to 3600 seconds on one router and 7200 seconds on the peer.
Why wrong: Incorrect. IPsec SA lifetimes are negotiated; a mismatch may cause rekey issues but does not prevent initial tunnel establishment.
- E
The transform set on one router uses ESP-AES and the peer uses ESP-3DES.
Why wrong: Incorrect. Different transform sets may still allow negotiation if both sides support multiple algorithms; this alone does not guarantee failure.
300-410 IKEv2 authentication Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: iKEv2 authentication. 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.
Which TWO actions will prevent an IPsec site-to-site VPN tunnel from coming up when using IKEv2 and pre-shared keys? (Choose TWO.)
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 pre-shared key configured on one router does not match the key on the peer.
Options A and B are correct. A: IKEv2 uses pre-shared keys for authentication during IKE_SA_INIT; a mismatch causes authentication failure and prevents tunnel establishment. B: Blocking UDP port 500 blocks IKE initial traffic, so no IKE SA can be formed. Option E is incorrect because IKEv2 negotiates encryption algorithms during IKE_AUTH; if both sides offer multiple transforms, a common algorithm can be selected. A single mismatch does not necessarily prevent the tunnel.
Key principle: IKEv2 authentication
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 pre-shared key configured on one router does not match the key on the peer.
Why this is correct
Correct. PSK mismatch causes IKE authentication to fail, preventing the tunnel from coming up.
Related concept
IKEv2 authentication
- ✓
An ACL on the router's outside interface blocks UDP port 500.
Why this is correct
Correct. UDP port 500 is required for IKE initial exchange; blocking it prevents IKE from starting.
Related concept
IKEv2 authentication
- ✗
An ACL on the router's outside interface blocks UDP port 4500.
- ✗
The IPsec SA lifetime is set to 3600 seconds on one router and 7200 seconds on the peer.
Why it's wrong here
Incorrect. IPsec SA lifetimes are negotiated; a mismatch may cause rekey issues but does not prevent initial tunnel establishment.
- ✗
The transform set on one router uses ESP-AES and the peer uses ESP-3DES.
Why it's wrong here
Incorrect. Different transform sets may still allow negotiation if both sides support multiple algorithms; this alone does not guarantee failure.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Common traps: thinking that blocking UDP 4500 alone will stop the tunnel (it won't unless NAT is used), and that different transform sets always prevent the tunnel (IKEv2 can negotiate a match if multiple proposals are configured).
Detailed technical explanation
How to think about this question
IKEv2 uses a two-exchange process: IKE_SA_INIT (for cryptographic suites and Diffie-Hellman) and IKE_AUTH (for identity and authentication). Pre-shared key authentication occurs in IKE_AUTH, where both peers compute a hash using the PSK; a mismatch causes an AUTHENTICATION_FAILED notification. For transform sets, IKEv2 requires an exact match of the encryption and integrity algorithms in the proposal; unlike IKEv1, there is no fallback or multiple proposal acceptance—if the first proposal fails, the entire negotiation fails. In real-world scenarios, misconfigured PSKs are common due to typos or copy-paste errors, while transform set mismatches often occur when upgrading hardware with different crypto engine capabilities.
KKey Concepts to Remember
- IKEv2 authentication
- UDP port 500
- IKEv2 negotiation
- NAT-T (UDP 4500)
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
IKEv2 authentication
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.
Review iKEv2 authentication, then practise related 300-410 questions on the same topic to reinforce the concept.
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPsec Site-to-Site VPN — This question tests IPsec Site-to-Site VPN — IKEv2 authentication.
What is the correct answer to this question?
The correct answer is: The pre-shared key configured on one router does not match the key on the peer. — Options A and B are correct. A: IKEv2 uses pre-shared keys for authentication during IKE_SA_INIT; a mismatch causes authentication failure and prevents tunnel establishment. B: Blocking UDP port 500 blocks IKE initial traffic, so no IKE SA can be formed. Option E is incorrect because IKEv2 negotiates encryption algorithms during IKE_AUTH; if both sides offer multiple transforms, a common algorithm can be selected. A single mismatch does not necessarily prevent the tunnel.
What should I do if I get this 300-410 question wrong?
Review iKEv2 authentication, then practise related 300-410 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
IKEv2 authentication
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Last reviewed: Jun 24, 2026
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