300-410 Infrastructure Services Practice Question
A network administrator is configuring a Cisco IOS router to authenticate users via TACACS+ using a TACACS+ server at 10.1.1.50. The administrator wants to ensure that if the TACACS+ server is unreachable, the router will fall back to using the local username database. Which command set achieves this?
⚠ Common exam trap
The trap here is reversing the order of authentication methods, which would cause the router to check local credentials first and only use TACACS+ if local fails, contrary to the desired primary-backup relationship.
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 aaa authentication login default group tacacs+ local command sets TACACS+ as the primary authentication method and local as the fallback. This ensures that if the TACACS+ server is unreachable, the router will use its local username database, maintaining administrative access without compromising security.
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 login authentication method list to first try TACACS+ and then fall back to the local database if the TACACS+ server is unreachable. The 'local' keyword ensures that local authentication is attempted only if the TACACS+ servers do not respond, providing redundancy and preventing lockout.
- ✗
aaa authentication login default group tacacs+ none
Why it's wrong here
The 'none' keyword means that if TACACS+ authentication fails or is unreachable, the user is granted access without any further authentication. This is a severe security risk because it allows unauthenticated access when the TACACS+ server is down. The requirement is to fall back to local authentication, not to bypass authentication entirely.
- ✗
aaa authentication login default group tacacs+ enable
Why it's wrong here
The 'enable' keyword uses the enable password as a fallback method. While this provides some authentication, it does not use the local username database. The requirement specifically states to fall back to the local username database, so 'enable' is not appropriate. Also, using the enable password for login authentication is less secure and not scalable.
- ✗
aaa authentication login default local group tacacs+
Why it's wrong here
This command lists 'local' before 'tacacs+', meaning the router will check the local database first and only use TACACS+ if local authentication fails. This is the opposite of the desired behavior; TACACS+ should be the primary method with local as backup. The order of methods in the list is critical.
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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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
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