hardMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting a router that is not allowing…
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?
⚠ Common exam trap
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.
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.
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.
- ✗
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.
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 |
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.