This KB exhibits a non service impact alarm "Fan Spinning Underspeed" and it provides the PR with the fix releases on EX4100s.

Symptoms

user@> show system information

Model: ex4100-48p

Family: junos-ex

Junos: 23.4R2-S4.11

Hostname: user

You might encounter the following alarm:

user@> show chassis alarms

1 alarms currently active

Alarm time Class Description

2025-04-22 02:55:03 UTC Minor FPC 4 Fan 0 Spinning Underspeed

And the following logs:

Apr 22 02:55:03 user alarmd[21938]: Alarm set: FAN id=100673680, color=YELLOW, class=CHASSIS, reason=FPC 4 Fan 0 Spinning Underspeed

Apr 22 02:55:03 user craftd[22569]: Receive FX craftd set alarm message: color: 2 class: 100 object: 144 slot: 40 silent: 0 short_reason=FPC 4 Fan 0 Underspeed long_reason=FPC 4 Fan 0 Spinning Underspeed id=100673680 reason=100663296

Apr 22 02:55:03 user craftd[22569]: Minor alarm set, FPC 4 Fan 0 Spinning Underspeed

When checking the fans, we can see that fan 0 of FPC4 is spinning slightly slower compared to the other fan trays. However, it is still showing as "Spinning at normal speed":

user@> show chassis fan

Item Status RPM Measurement

FPC 0 Fan Tray 0 Fan 0 OK 4920 Spinning at normal speed

FPC 0 Fan Tray 1 Fan 0 OK 5040 Spinning at normal speed

FPC 1 Fan Tray 0 Fan 0 OK 4680 Spinning at normal speed

FPC 1 Fan Tray 1 Fan 0 OK 4800 Spinning at normal speed

FPC 2 Fan Tray 0 Fan 0 OK 4800 Spinning at normal speed

FPC 2 Fan Tray 1 Fan 0 OK 5040 Spinning at normal speed

FPC 3 Fan Tray 0 Fan 0 OK 4680 Spinning at normal speed

FPC 3 Fan Tray 1 Fan 0 OK 4560 Spinning at normal speed

FPC 4 Fan Tray 0 Fan 0 OK 4320 Spinning at normal speed

FPC 4 Fan Tray 1 Fan 0 OK 5160 Spinning at normal speed

This issue does not impact the switch services or health. It is related to incorrect data being sent to the chassis by the responsible process, causing the alarm to be constantly raised and cleared. The fan underspeed alarms may intermittently trigger as fan speeds fall below the set threshold, but there is no impact on service. These alarms are addressed in PR1848292 and resolved in the following version:

Resolved In

Junos
22.4R3-S6
23.2R2-S4
23.4R2-S4
24.2R2
24.4R1
24.4R2
25.2R1

https://prsearch.juniper.net/problemreport/PR1848292