- A
The pre-shared key configured on one router does not match the key on the peer.
Correct. Authentication will fail, and the IKE SA will not be established.
- B
An ACL on the router's outside interface blocks UDP port 500.
Correct. IKE uses UDP 500; blocking it prevents IKE negotiation.
- C
An ACL on the router's outside interface blocks UDP port 4500.
Why wrong: Incorrect. UDP 4500 is used for NAT traversal, but initial IKE uses UDP 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 lifetime mismatch does not prevent the tunnel from coming up; it only affects rekey timing.
- E
The transform set on one router uses ESP-AES and the peer uses ESP-3DES.
Correct. Transform sets must match exactly; otherwise, IPsec SA negotiation fails.
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site 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.
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.
Option A is correct because IKEv2 uses pre-shared keys (PSK) for authentication during the IKE_SA_INIT exchange. If the PSK configured on one router does not match the key on the peer, the authentication check fails, and the IKEv2 session is terminated before any IPsec SAs can be established. This mismatch directly prevents the VPN tunnel from coming up.
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 pre-shared key configured on one router does not match the key on the peer.
Why this is correct
Correct. Authentication will fail, and the IKE SA will not be established.
Related concept
Read the scenario before looking for a memorised answer.
- ✓
An ACL on the router's outside interface blocks UDP port 500.
Why this is correct
Correct. IKE uses UDP 500; blocking it prevents IKE negotiation.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
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 lifetime mismatch does not prevent the tunnel from coming up; it only affects rekey timing.
- ✓
The transform set on one router uses ESP-AES and the peer uses ESP-3DES.
Why this is correct
Correct. Transform sets must match exactly; otherwise, IPsec SA negotiation fails.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that blocking UDP port 4500 alone will prevent the tunnel, but the trap is that NAT-T uses port 4500 only after detecting NAT, and the initial IKE exchange always requires UDP port 500, so blocking only port 4500 does not stop the tunnel unless NAT is present and the router fails to negotiate port 500 fallback.
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
- 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 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.
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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 — Read the scenario before looking for a memorised answer..
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. — Option A is correct because IKEv2 uses pre-shared keys (PSK) for authentication during the IKE_SA_INIT exchange. If the PSK configured on one router does not match the key on the peer, the authentication check fails, and the IKEv2 session is terminated before any IPsec SAs can be established. This mismatch directly prevents the VPN tunnel from coming up.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
This 300-410 practice question is part of Courseiva's free Cisco 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 300-410 exam.
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