This 300-410 practice question tests your understanding of eigrp troubleshooting. 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 engineer configures IPsec between two routers using a site-to-site VPN with IKEv1. The configuration uses `crypto isakmp policy 10` with authentication pre-share and encryption aes. On the peer, the policy is configured with authentication pre-share and encryption 3des. Unexpectedly, the IKE phase 1 negotiation fails. Which is the most likely explanation?
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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The encryption algorithms (AES vs 3DES) do not match, causing IKE phase 1 to fail.
IKEv1 requires that the encryption algorithm, hash, authentication method, and Diffie-Hellman group match exactly between peers. The encryption algorithm mismatch (AES vs 3DES) causes the IKE proposal to be rejected. Even though both use pre-shared keys, the encryption mismatch is a common edge case.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 encryption algorithms (AES vs 3DES) do not match, causing IKE phase 1 to fail.
Why this is correct
IKEv1 requires exact match of all parameters in the proposal.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The pre-shared key must be configured globally, not under the policy.
Why it's wrong here
Pre-shared key is configured with `crypto isakmp key` command, not under the policy.
✗
The IKE policy must have the same priority number on both ends.
Why it's wrong here
Priority numbers are local and do not need to match.
✗
The authentication method must be `rsa-sig` for site-to-site VPNs.
Why it's wrong here
Pre-shared key is valid for site-to-site VPNs.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Command / output trap
Pre-shared key is configured with `crypto isakmp key` command, not under the policy.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The encryption algorithms (AES vs 3DES) do not match, causing IKE phase 1 to fail. — IKEv1 requires that the encryption algorithm, hash, authentication method, and Diffie-Hellman group match exactly between peers. The encryption algorithm mismatch (AES vs 3DES) causes the IKE proposal to be rejected. Even though both use pre-shared keys, the encryption mismatch is a common edge case.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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