SystemCCNA 200-301

NTP Time Offset Too Large — AAA Authentication Failing

Presenting Symptom

Users are unable to authenticate via RADIUS/TACACS+, and AAA login attempts fail with 'Authentication failed' errors.

Network Context

A small branch office with a Cisco ISR 4321 router (IOS XE 16.9) acting as the network gateway and AAA client. The router is configured to authenticate management access (SSH, console) via a remote AAA server (Cisco ISE) using RADIUS. The router also runs NTP client pointing to the same ISE server as the NTP server. The network has a single LAN segment with a few switches and a handful of users.

Diagnostic Steps

Check AAA authentication status and debug

show aaa servers

RADIUS: id 1, priority 1, host 192.168.1.10, auth-port 1812, acct-port 1813
  State: current UP, duration 0s, previous duration 0s
  Dead: total 0, retries 0, timeout 5
  ...

Look for the RADIUS server state. If it shows 'current UP' but authentication still fails, the issue is not server reachability. If it shows 'current DOWN' or 'DEAD', check connectivity or shared secret.

Check NTP synchronization status

show ntp status

Clock is synchronized, stratum 2, reference is 192.168.1.10
  nominal freq is 250.0000 Hz, actual freq is 249.9990 Hz, precision is 2**10
  ntp uptime is 3600 seconds, resolution is 4000
  ...

If the clock is not synchronized (e.g., 'Clock is unsynchronized'), NTP is not working. Even if synchronized, note the time offset.

Check NTP associations and offset

show ntp associations detail

192.168.1.10 configured, our_master, sane, valid, stratum 2
  ref ID 10.0.0.1, time D6C8E1A0.12345678 (01:23:45.567 UTC Mon Mar 1 2021)
  our mode client, peer mode server, our poll intvl 64, peer poll intvl 64
  root delay 10.00 msec, root dispersion 5.00 msec, offset 5000.00 msec
  ...

Look at the 'offset' field. An offset greater than 1000 msec (1 second) is problematic for AAA. The example shows 5000 msec (5 seconds), which is too large. Normal offset should be less than 100 msec.

Verify AAA debug logs for time-related errors

debug radius authentication

RADIUS: authenticating to get author for 192.168.1.10
  RADIUS: Received from id 1 192.168.1.10:1812, Access-Reject, len 20
  RADIUS: Authentication return Access-Reject
  ...

If you see 'Access-Reject' without any other error, the AAA server is rejecting due to time offset. Check the AAA server logs for 'time offset too large' or 'clock skew' messages.

Root Cause

The router's system clock has a large offset (5000 ms) from the NTP server (which is also the AAA server). AAA protocols like RADIUS and TACACS+ require time synchronization within a small tolerance (typically < 300 seconds, but often stricter). The large NTP offset causes the AAA server to reject authentication requests because the timestamps in the packets are outside the acceptable window.

Resolution

1. Correct the NTP configuration to ensure accurate time synchronization: - Remove the existing NTP server if misconfigured: 'no ntp server 192.168.1.10' - Add a reliable NTP server (e.g., pool.ntp.org): 'ntp server pool.ntp.org' - Alternatively, if using the same server, ensure the server itself has correct time. 2. Manually set the clock to a reasonable value if NTP is not immediately available: 'clock set 12:00:00 Mar 1 2021' 3. If the offset is due to timezone misconfiguration, set the correct timezone: 'clock timezone EST -5' and 'clock summer-time EDT recurring' 4. Wait for NTP synchronization: 'show ntp status' should show 'Clock is synchronized' and offset < 100 ms.

Verification

1. 'show ntp status' — expect 'Clock is synchronized' with stratum < 16. 2. 'show ntp associations detail' — expect offset less than 100 msec. 3. 'show aaa servers' — expect RADIUS server state 'current UP'. 4. Test AAA authentication: 'test aaa group radius legacy user password' — expect 'Authentication successful'.

Prevention

1. Use a dedicated, reliable NTP server (e.g., from a trusted pool or internal stratum 1 server) rather than relying on the AAA server for NTP. 2. Configure NTP authentication to prevent rogue NTP servers from skewing the clock. 3. Regularly monitor NTP offset using SNMP or syslog alerts for offsets exceeding 100 ms.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why AAA authentication fails. The exam tests understanding that AAA protocols rely on accurate time synchronization. A common multiple-choice question might show 'show ntp associations detail' output with a large offset and ask for the root cause. Key fact: NTP offset > 1000 ms can cause AAA failures.

Exam Tips

Memorize that RADIUS and TACACS+ require time synchronization; a large NTP offset is a common cause of authentication failures.

In the exam, if you see 'Access-Reject' in debug output, check NTP offset before assuming shared secret issues.

Know the command 'show ntp associations detail' and how to interpret the 'offset' field.

Commands Used in This Scenario

show aaa servers

Displays the status and statistics of all configured AAA (Authentication, Authorization, and Accounting) servers, used to verify server reachability and authentication activity.

show ntp status

Displays the current NTP synchronization status, including clock stratum, reference clock, and synchronization state, used to verify NTP operation and clock accuracy.

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