- A
The RSA key was generated with a modulus less than 768 bits, causing SSH to fail.
Cisco IOS requires a minimum 768-bit RSA key for SSHv2; if the key is smaller, SSH will not accept connections.
- B
The VTY lines are missing the 'login local' command.
Why wrong: The stem states 'login local' is configured, so this is not the issue.
- C
The router has 'ip ssh version 1' configured, and the client only supports SSHv2.
Why wrong: While possible, the default is SSHv2; the engineer would need to explicitly configure version 1, which is less common.
- D
The management station is using the wrong username or password.
Why wrong: If the RSA key is valid, SSH would attempt authentication; the symptom of no connection at all suggests a transport layer issue, not authentication.
300-410 Device Management Practice Question
This 300-410 practice question tests your understanding of device management. 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.
A network engineer is troubleshooting a router that is not allowing SSH connections from any management station. The router has 'ip domain-name example.com' and 'crypto key generate rsa' configured. 'line vty 0 4' has 'transport input ssh' and 'login local'. The engineer can ping the router from a management station. What is the most likely cause?
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 RSA key was generated with a modulus less than 768 bits, causing SSH to fail.
The most likely cause is that the RSA key was generated with a modulus less than 768 bits. Cisco IOS requires an RSA key modulus of at least 768 bits for SSH to function; if the modulus is smaller (e.g., 512 bits), SSH will fail to start even though the key is generated successfully. The presence of 'ip domain-name', 'crypto key generate rsa', 'transport input ssh', and 'login local' on the VTY lines, combined with successful ping reachability, points to an SSH-specific configuration issue rather than connectivity or authentication problems.
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 RSA key was generated with a modulus less than 768 bits, causing SSH to fail.
Why this is correct
Cisco IOS requires a minimum 768-bit RSA key for SSHv2; if the key is smaller, SSH will not accept connections.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The VTY lines are missing the 'login local' command.
Why it's wrong here
The stem states 'login local' is configured, so this is not the issue.
- ✗
The router has 'ip ssh version 1' configured, and the client only supports SSHv2.
Why it's wrong here
While possible, the default is SSHv2; the engineer would need to explicitly configure version 1, which is less common.
- ✗
The management station is using the wrong username or password.
Why it's wrong here
If the RSA key is valid, SSH would attempt authentication; the symptom of no connection at all suggests a transport layer issue, not authentication.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the specific modulus requirement for SSH (768 bits minimum) and the fact that a successful 'crypto key generate rsa' does not guarantee SSH will work if the key is too small, leading candidates to overlook this subtle but critical detail.
Detailed technical explanation
How to think about this question
When 'crypto key generate rsa' is executed without specifying a modulus, Cisco IOS defaults to 512 bits on some older platforms, which is insufficient for SSH. SSHv2 requires a minimum key size of 768 bits (per RFC 4253), and Cisco enforces this by refusing to start the SSH server if the RSA key is too small. A real-world scenario is a network engineer who generates keys quickly without specifying the modulus, then spends hours troubleshooting why SSH fails despite all other configurations being correct.
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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Quick reference
Asymmetric Encryption Algorithm Comparison
| Algorithm | Key Exchange | Signatures | Equivalent Security Key | Notes |
|---|---|---|---|---|
| RSA-3072 | Yes | Yes | 128-bit | Widely deployed; slow for bulk data |
| ECDSA P-256 | No | Yes | 128-bit | Fast signatures; standard TLS certs |
| ECDH / ECDHE | Yes | No | 128-bit | Perfect forward secrecy in TLS 1.3 |
| DH / DHE | Yes | No | 128-bit (3072-bit key) | Replaced by ECDHE in modern TLS |
| Ed25519 | No | Yes | ~128-bit | SSH keys, modern PKI |
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?
Device Management — This question tests Device Management — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The RSA key was generated with a modulus less than 768 bits, causing SSH to fail. — The most likely cause is that the RSA key was generated with a modulus less than 768 bits. Cisco IOS requires an RSA key modulus of at least 768 bits for SSH to function; if the modulus is smaller (e.g., 512 bits), SSH will fail to start even though the key is generated successfully. The presence of 'ip domain-name', 'crypto key generate rsa', 'transport input ssh', and 'login local' on the VTY lines, combined with successful ping reachability, points to an SSH-specific configuration issue rather than connectivity or authentication problems.
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.
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.
About these practice questions
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Last reviewed: Jul 4, 2026
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