350-601 Security Practice Question
An administrator configures 'aaa authentication login default group tacacs+ local'. What happens if the TACACS+ server is unreachable?
⚠ Common exam trap
Cisco often tests the distinction between a server being unreachable (fallback occurs) versus a server rejecting credentials (authentication fails immediately), causing candidates to incorrectly assume that any TACACS+ issue results in authentication failure.
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
✓
Local username database is used
The command 'aaa authentication login default group tacacs+ local' configures a method list where the first method is TACACS+ and the second is local. If the TACACS+ server is unreachable (not responding, not rejecting), the switch falls back to the next method in the list, which is local authentication using the local username database. This fallback behavior is defined by Cisco IOS/IOS-XE authentication method lists, where 'local' acts as a backup when the primary method is unavailable.
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 switch uses no authentication
Why it's wrong here
Authentication always uses a method; local is used if TACACS+ is unreachable.
- ✗
Authentication fails
Why it's wrong here
The fallback to local prevents immediate failure.
- ✗
The switch tries the next method in the group
Why it's wrong here
The group is tried first; if it fails, only then local is used, not within the same group.
- ✓
Local username database is used
Why this is correct
The config includes 'local' as a fallback method after group tacacs+.
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 by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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