Redundant Power Supply Failure — No Alert Generated
Presenting Symptom
A network engineer notices that a critical switch in the data center is running on a single power supply, but no alert or syslog message was generated when the redundant power supply failed.
Network Context
This occurs in a small branch office with a Cisco Catalyst 2960-X switch running IOS 15.2. The switch has dual power supplies (PSU1 and PSU2) configured for redundancy. The network is managed via SNMP and syslog, but no alert was received when PSU2 failed. The switch continues to operate normally on PSU1, but the failure went unnoticed until a physical inspection.
Diagnostic Steps
Check power supply status
show env powerPS1: OK PS2: NOT PRESENT
The output shows PS2 as 'NOT PRESENT' instead of 'OK' or 'FAIL'. This indicates the power supply is either physically removed or failed without being detected. If it were detected as failed, it would show 'FAIL'.
Check environmental monitoring configuration
show env allPower Supply 1: OK Power Supply 2: NOT PRESENT Fan 1: OK Fan 2: OK Temperature: OK Environmental monitoring: Enabled
This confirms that environmental monitoring is enabled, but the power supply status shows 'NOT PRESENT' instead of 'FAIL'. This suggests the switch did not detect the failure as a critical event.
Check syslog configuration for power events
show logging | include ENVMON|POWERNo output (no matching lines)
No syslog messages related to power or environmental monitoring were generated. This indicates that the switch did not log the power supply failure, possibly because the failure was not detected as a state change (e.g., the power supply was removed without the switch noticing).
Check SNMP traps configuration
show snmp | include trap|powerSNMP traps: enabled Trap destination: 192.168.1.100 Community: public No specific power trap configuration shown
SNMP traps are enabled, but there is no specific configuration for power supply traps. By default, Cisco switches may not send traps for power supply failures unless explicitly configured with 'snmp-server enable traps envmon'.
Root Cause
The power supply failure was not detected because the switch's environmental monitoring did not generate an alert. This is due to the fact that the power supply was physically removed or failed in a way that the switch did not register as a state change (e.g., the power supply was unplugged while the switch was off, or the power supply's internal circuitry failed without triggering the monitoring circuit). Additionally, SNMP traps for environmental monitoring were not enabled, so no alert was sent to the NMS.
Resolution
Verification
Run 'show env power' and verify both power supplies show 'OK'. Run 'show logging | include ENVMON' and verify syslog messages like '%ENVMON-4-POWERSUPPLY' appear. Run 'show snmp' and confirm traps are enabled for envmon.
Prevention
1. Always enable SNMP traps for environmental monitoring using 'snmp-server enable traps envmon'. 2. Configure syslog to log all environmental events with 'logging buffered 10000 debugging' or similar. 3. Regularly inspect power supply status via 'show env power' as part of routine maintenance.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario tests knowledge of power supply redundancy and environmental monitoring. Expect multiple-choice questions asking which command enables SNMP traps for power supply failures, or troubleshooting questions where you must identify why no alert was generated. Key fact: 'snmp-server enable traps envmon' is required to send power supply failure traps.
Exam Tips
Remember that 'show env power' displays power supply status, and 'NOT PRESENT' means the switch doesn't detect the power supply.
The default SNMP trap configuration does not include environmental traps; you must explicitly enable them with 'snmp-server enable traps envmon'.
Syslog messages for power failures start with '%ENVMON-4-POWERSUPPLY' — memorize this pattern.
Commands Used in This Scenario
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