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[SRX] Different fan speeds displayed on nodes even though the fan RPM is similar

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Article ID: KB34901 KB Last Updated: 27 Aug 2019Version: 1.0
Summary:

This article explains why different fan spin speeds are displayed on SRX5800 device nodes even though the RPMs are similar, and indicates whether any action is required to correct the speeds.

Symptoms:

Observe in the following output that node0's RPM is about 4600 and the fan is "spinning at normal speed". On node1, on the other hand, the RPM is similar but the fan is said to be "spinning at high speed". This is seen to happen randomly.

root@SRX5800> show chassis fan
node0:
--------------------------------------------------------------------------
      Item                      Status   RPM     Measurement
      Top Tray Fan 1            OK       4637    Spinning at normal speed
      Top Tray Fan 2            OK       4637    Spinning at normal speed
      Top Tray Fan 3            OK       4616    Spinning at normal speed
      Top Tray Fan 4            OK       4637    Spinning at normal speed
      Top Tray Fan 5            OK       4616    Spinning at normal speed
      Top Tray Fan 6            OK       4637    Spinning at normal speed
      Top Tray Fan 7            OK       4637    Spinning at normal speed
      Top Tray Fan 8            OK       4616    Spinning at normal speed
      Top Tray Fan 9            OK       4616    Spinning at normal speed
      Top Tray Fan 10           OK       4616    Spinning at normal speed
      Top Tray Fan 11           OK       4637    Spinning at normal speed
      Top Tray Fan 12           OK       4616    Spinning at normal speed
      Bottom Tray Fan 1         OK       4595    Spinning at normal speed
      Bottom Tray Fan 2         OK       4616    Spinning at normal speed
      Bottom Tray Fan 3         OK       4616    Spinning at normal speed
      Bottom Tray Fan 4         OK       4637    Spinning at normal speed
      Bottom Tray Fan 5         OK       4616    Spinning at normal speed
      Bottom Tray Fan 6         OK       4616    Spinning at normal speed
      Bottom Tray Fan 7         OK       4658    Spinning at normal speed
      Bottom Tray Fan 8         OK       4616    Spinning at normal speed
      Bottom Tray Fan 9         OK       4616    Spinning at normal speed
      Bottom Tray Fan 10        OK       4616    Spinning at normal speed
      Bottom Tray Fan 11        OK       4616    Spinning at normal speed
      Bottom Tray Fan 12        OK       4616    Spinning at normal speed

node1:
--------------------------------------------------------------------------
      Item                      Status   RPM     Measurement
      Top Tray Fan 1            OK       4637    Spinning at high speed
      Top Tray Fan 2            OK       4637    Spinning at high speed
      Top Tray Fan 3            OK       4616    Spinning at high speed
      Top Tray Fan 4            OK       4637    Spinning at high speed
      Top Tray Fan 5            OK       4637    Spinning at high speed
      Top Tray Fan 6            OK       4616    Spinning at high speed
      Top Tray Fan 7            OK       4637    Spinning at high speed
      Top Tray Fan 8            OK       4616    Spinning at high speed
      Top Tray Fan 9            OK       4637    Spinning at high speed
      Top Tray Fan 10           OK       4616    Spinning at high speed
      Top Tray Fan 11           OK       4637    Spinning at high speed
      Top Tray Fan 12           OK       4616    Spinning at high speed
      Bottom Tray Fan 1         OK       4595    Spinning at high speed
      Bottom Tray Fan 2         OK       4616    Spinning at high speed
      Bottom Tray Fan 3         OK       4637    Spinning at high speed
      Bottom Tray Fan 4         OK       4616    Spinning at high speed
      Bottom Tray Fan 5         OK       4616    Spinning at high speed
      Bottom Tray Fan 6         OK       4637    Spinning at high speed
      Bottom Tray Fan 7         OK       4637    Spinning at high speed
      Bottom Tray Fan 8         OK       4616    Spinning at high speed
      Bottom Tray Fan 9         OK       4637    Spinning at high speed
      Bottom Tray Fan 10        OK       4616    Spinning at high speed
      Bottom Tray Fan 11        OK       4616    Spinning at high speed
      Bottom Tray Fan 12        OK       4637    Spinning at high speed

Cause:

On SRX5800 devices, if the state of a fan is "spinning at high speed" and the temperature of the chassis is equal to or below 55 degrees, the state will be changed to "spinning at normal speed." Otherwise, the state of the fan will be "spinning at high speed."

If the state of the fan is "spinning at normal speed" and the temperature of the chassis is equal to or below 60 degrees, the state will be "spinning at normal speed." Otherwise, the state is changed to "spinning at high speed."

Note that there is correlation between fan speed (RPM value) and temperature, but no one-to-one relationship between temperature and spinning speeds. There is, however, an overlapping range between 55 degrees and 60 degrees temperature.

Therefore, the spinning speed could be either "high" or "normal" depending on the previous speed state. At some time, the temperature of node 1 may be above 60 degrees, so the state is converted to "spinning at high speed." But the temperature is never below 55 degrees, so the state remains at "spinning at high speed."

Only the following three temperature sensors on the FPC are used for fan speed control.

  • Intake

  • ExhaustA

  • ExhaustB

 

Example

root@SRX5800> show chassis environment fpc 0
FPC 0 status:
  State                      Online
  Temperature Intake         37 degrees C / 98 degrees F            
  Temperature Exhaust A      49 degrees C / 120 degrees F           
  Temperature Exhaust B      59 degrees C / 138 degrees F 
Solution:

This behavior is as per current design and no action is required because it is not an indication of a problem.

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