Depending on the ambient conditions, the system optimises the fan-speed for temperature and pressure and
maintains the minimum fan speeds and temperatures that the chassis requires. Accordingly, the fan tray's
internal controller operates in a normal mode and a Network Equipment-Building System mode (NEBS mode).
• In the normal mode—the controller optimizes fan-speed for temperature and pressure.
• In the NEBS mode—the controller responds more aggressively, to maintain proper cooling for installations
with high airflow impedance. When the fan tray operates in this mode, there may be increased noise
levels due to increased fan speeds.
You can also manually set the fan tray to operate in the NEBS mode.
High Availability
To ensure high availability, the system is designed to respond to fan failures by either minimising impact or
by compensating and operating at a worst case scenario specification.
• If a single fan fails, the remaining fans in the row compensate with increased speed.
• If two or more fans fail, the entire fan tray operates at full speed after one minute. Further, the fan tray
must be replaced. The individual fans are not field replaceable, you must replace the fan tray.
• If the temperature sensor fails or communication with the temperature sensor fails, the worst case operating
temperature is assumed.
• If the pressure sensor fails or communication with the pressure sensor fails, the worst case operating
pressure is assumed.
Thresholds, Alarms, and Abnormal Acoustic Conditions
Under normal operating conditions, when none of the temperature alarms have been triggered, hardware
controls fan speed. If any of the system temperature alarms are triggered, software takes fan speed control
from hardware. Refer to the System Management section of the software configuration guide for more
information.
When the fan tray operates at full speed, increased noise levels may be expected. The fan tray may operate
at full speed in these circumstances:
• If the fan tray is not installed correctly
• When operating in the NEBS mode
• If two or more fans have failed
• If the STATUS LED remains red for more than one minute
• If there is a hardware failure
• If the software watchdog timer is triggered
Cisco Catalyst 9400 Series Switches Hardware Installation Guide
19
Product Overview
High Availability
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