300-410 Infrastructure Security Practice Question
A network engineer is configuring a Cisco IOS router to authenticate administrative SSH logins against an external TACACS+ server. The engineer wants to ensure that if the TACACS+ server becomes unreachable, local authentication is used as a fallback. Which configuration accomplishes this?
⚠ Common exam trap
The trap here is assuming that any fallback method is acceptable, but the order of methods in the AAA authentication command determines which is primary and which is backup.
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
✓
aaa authentication login default group tacacs+ local
The correct command is 'aaa authentication login default group tacacs+ local'. It configures the default method list to try TACACS+ first and then the local database if the server is unreachable. This ensures administrative SSH logins are authenticated externally when possible, but local fallback maintains access during server outages. The order of methods is critical: listing 'local' first would bypass the TACACS+ server.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
aaa authentication login default group tacacs+ local
Why this is correct
This command configures the default method list to first attempt TACACS+ authentication and then fall back to the local username database if the TACACS+ server is unreachable. The 'group tacacs+' keyword specifies the TACACS+ server group, and 'local' provides the backup method. This is the correct way to ensure administrative access is not lost when the external server fails.
- ✗
aaa authentication login default group radius local
Why it's wrong here
This command uses RADIUS instead of TACACS+, which is a different protocol. Although it includes local fallback, it does not satisfy the requirement to authenticate against a TACACS+ server. TACACS+ and RADIUS are not interchangeable; the engineer explicitly wants TACACS+, so this configuration is wrong for the scenario.
- ✗
aaa authentication login default local group tacacs+
Why it's wrong here
This command reverses the order: it attempts local authentication first and only uses TACACS+ if local authentication fails or is unavailable. In this scenario, the engineer wants TACACS+ to be the primary method, with local as fallback. Therefore, this order does not meet the requirement and would not use the external server for normal authentication.
- ✗
aaa authentication login default group tacacs+ none
Why it's wrong here
The 'none' keyword disables authentication entirely if the TACACS+ server is unreachable, allowing access without any credentials. This is a severe security risk and does not provide local fallback. The engineer specifically wants local authentication as a backup, not unrestricted access, so this configuration is incorrect and dangerous.
Quick reference
AAA Protocol Comparison
| Protocol | Port(s) | Encryption | Transport | Primary Use |
|---|---|---|---|---|
| RADIUS | 1812 / 1813 | Password only | UDP | Network access control |
| TACACS+ | 49 | Full packet | TCP | Device administration |
| Diameter | 3868 | Full session | TCP / SCTP | Carrier / mobile networks |
| 802.1X | — | EAP-based | Layer 2 | Port-based access control |
TACACS+ encrypts the entire packet; RADIUS only encrypts the password field — a key exam distinction.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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.