TACACS+ Authentication Failing for All Users
Presenting Symptom
All users are unable to authenticate via TACACS+ for device administration, and local fallback authentication also fails.
Network Context
A small enterprise branch office with a Cisco ISR 4331 router running IOS XE 16.9. The router is configured for AAA authentication using a single TACACS+ server at 192.168.1.100. The network has about 50 users and 10 network devices. The TACACS+ server is a Linux-based server running freeTACACS.
Diagnostic Steps
Check AAA configuration and TACACS+ server status
show running-config | section aaaaaa new-model aaa authentication login default group tacacs+ local aaa authorization exec default group tacacs+ local tacacs-server host 192.168.1.100 key cisco123
Verify that AAA is enabled and TACACS+ is configured as the primary authentication method with local fallback. Ensure the server IP and key are correct.
Test connectivity to TACACS+ server
ping 192.168.1.100!!!!! (all five pings successful)
If ping fails, there is a Layer 3 connectivity issue. If ping succeeds, the problem is likely with the TACACS+ protocol or server configuration.
Verify TACACS+ server reachability on port 49
telnet 192.168.1.100 49Trying 192.168.1.100, 49 ... Open (connection established)
If connection fails, the TACACS+ server may not be listening on port 49 or a firewall is blocking it. If successful, the issue is likely with authentication credentials or server configuration.
Enable TACACS+ debugging to see authentication attempts
debug tacacsTACACS: Opening TCP/IP connection to 192.168.1.100/49 TACACS: Sending AUTHEN/START packet TACACS: Received AUTHEN/REPLY packet with status FAIL
If you see 'FAIL' status, the server is rejecting the authentication. If you see 'ERROR' or no response, there may be a server or network issue. Look for the specific failure reason.
Check TACACS+ server logs
On the TACACS+ server: tail -f /var/log/tac_plus.logFailed authentication for user 'admin' from 192.168.1.1: Invalid password
Server logs will show the exact reason for authentication failure, such as invalid password, unknown user, or misconfigured shared secret.
Root Cause
The TACACS+ server's shared secret key does not match the key configured on the router. The router has 'key cisco123' but the server expects 'Cisco123' (case-sensitive mismatch).
Resolution
Verification
Run 'debug tacacs' and attempt authentication again. Expected output: TACACS: Sending AUTHEN/START packet TACACS: Received AUTHEN/REPLY packet with status PASS Also verify with 'show tacacs' to see server statistics and successful authentications.
Prevention
1. Always document and securely share TACACS+ keys between devices and servers. 2. Use a centralized key management system or automation to ensure consistency. 3. Regularly audit AAA configurations and test authentication after any changes.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where AAA authentication fails. The exam tests understanding of AAA configuration, TACACS+ vs RADIUS, and the importance of matching shared secrets. Candidates must know the 'debug tacacs' command and how to interpret its output.
Exam Tips
Memorize the 'debug tacacs' command and its output to identify authentication failures.
Remember that TACACS+ uses TCP port 49 and encrypts the entire packet, while RADIUS uses UDP and encrypts only the password.
Know that the 'tacacs-server key' command must match exactly (case-sensitive) on both the router and the server.
Commands Used in This Scenario
ping [ip]
The ping command sends ICMP echo requests to a destination IP address to test network connectivity and measure round-trip time.
telnet [ip]
The telnet command establishes an unencrypted remote terminal session to another network device using the Telnet protocol, typically for remote management and troubleshooting.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
Practice CCNA Questions