Symptom

In Nexus 55xx switches, fans stop getting detected by the switch even though the fans are present in the system and spinning. if this condition is hit, messages such as following will be printed and the switch will shut down even though the fans are present and spinning.

2015 Mar 19 22:13:44.791 Nexus5548UP %$ VDC-1 %$ %NOHMS-2-NOHMS_ENV_ERR_FAN_DOWN: System minor alarm on fans: One fan missing or failed 2015 Mar 19 22:13:44.792 Nexus5548UP %$ VDC-1 %$ %PFMA-2-PFMA_FAN_REMOVED: Fan module 1 N5548P-FAN removed 2015 Mar 19 22:13:45.015 Nexus5548UP %$ VDC-1 %$ %PFMA-0-SYS_SHUTDOWN_FAN_REMOVAL: System shutdown in 60 seconds due to fan removal 2015 Mar 19 22:13:45.015 Nexus5548UP %$ VDC-1 %$ %NOHMS-2-NOHMS_ENV_ERR_FANS_DOWN: System major alarm on fans: Multiple fans missing or failed 2015 Mar 19 22:13:45.017 Nexus5548UP %$ VDC-1 %$ %PFMA-2-PFMA_FAN_REMOVED: Fan module 2 N5548P-FAN removed 2015 Mar 19 22:13:50.012 Nexus5548UP %$ VDC-1 %$ %PFMA-0-SYS_SHUTDOWN_FAN_REMOVAL: System shutdown in 55 seconds due to fan removal 2015 Mar 19 22:13:55.013 Nexus5548UP %$ VDC-1 %$ %PFMA-0-SYS_SHUTDOWN_FAN_REMOVAL: System shutdown in 50 seconds due to fan removal

Conditions

Unknown

Workaround

None

Further Problem Description

The problem is seen due to system micro-controller not seeing fans even though it records the fan speed RPM. As part of this bug fix, the fan detection logic has been made more robust by also looking at RPM readings.