The answer is that IKE phase 1 will fail due to encryption algorithm mismatch. This occurs because both peers must share a common ISAKMP policy proposal, and the encryption algorithm is a mandatory, non-negotiable parameter during the initial security association setup. Cisco IOS does not support automatic fallback or negotiation between different encryption types; if R1 offers AES 256 and the remote peer only offers 3DES, no matching proposal exists, causing the IKE SA to be rejected. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of IKE phase 1 negotiation requirements and the strict matching logic for encryption, hash, authentication, and Diffie-Hellman group. A common trap is assuming that stronger encryption will be accepted or that a fallback occurs, but Cisco devices require an exact match. Remember the mnemonic "E.A.G.L.E." for the five mandatory IKE policy parameters that must match: Encryption, Authentication, Group, Lifetime, and (pre-shared) Key—if any one is off, phase 1 fails.
300-410 IPsec Site-to-Site VPN Practice Question
This 300-410 practice question tests your understanding of ipsec site-to-site vpn. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Consider the following configuration on router R1:
crypto isakmp policy 10
encryption aes 256
authentication pre-share
group 14
lifetime 86400
!
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
IKE phase 1 will fail due to encryption algorithm mismatch.
IKE phase 1 requires both peers to have a matching ISAKMP policy, including the encryption algorithm. Since R1 is configured with AES 256 and the remote peer uses 3DES, there is no common proposal, causing phase 1 to fail. Cisco IOS does not automatically negotiate or fall back to a different encryption algorithm; the mismatch results in a failed IKE SA.
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 tunnel will use 3DES because the remote peer's policy is accepted.
Why it's wrong here
Both sides must have a matching policy; the initiator's policy list is sent, and the responder must match one.
✓
IKE phase 1 will fail due to encryption algorithm mismatch.
Why this is correct
The encryption algorithm must match; AES 256 vs 3DES is a mismatch.
Related concept
Read the scenario before looking for a memorised answer.
✗
The router will automatically adjust to use 3DES.
Why it's wrong here
No automatic adjustment; the configuration is static.
✗
The tunnel will establish but use AES 256 anyway.
Why it's wrong here
The responder must have a matching policy; otherwise, negotiation fails.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that IKE will automatically negotiate or fall back to a weaker algorithm, but in reality, IKE phase 1 requires an exact match of all policy parameters, and a mismatch causes the entire VPN to fail.
Detailed technical explanation
How to think about this question
IKE phase 1 uses a proposal exchange where each side sends its configured policies; the initiator's first proposal must match a responder's policy exactly (including encryption, hash, DH group, lifetime, and authentication method). If no match is found, the responder sends a 'no acceptable proposal' notification, and the IKE session terminates. This behavior is defined in RFC 2409 and RFC 7296 for IKEv1 and IKEv2, respectively.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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: IKE phase 1 will fail due to encryption algorithm mismatch. — IKE phase 1 requires both peers to have a matching ISAKMP policy, including the encryption algorithm. Since R1 is configured with AES 256 and the remote peer uses 3DES, there is no common proposal, causing phase 1 to fail. Cisco IOS does not automatically negotiate or fall back to a different encryption algorithm; the mismatch results in a failed IKE SA.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.