Description

This article explains the meaning of "PMBus Device Fail" syslog messages and how to resolve them.

For fixed releases, refer to PR1298612 - MX platforms may display false positive CB alarm "PMBus Device Fail" .

Symptoms

MX platforms may display false positive CB alarm, "PMBus Device Fail". They are usually set and then get cleared within a minute.

On MX platforms with Junos 15.1 release or above (with freeBSD 10), the following false positive CB alarms might be seen when RE is under stress: 
acb_pmbus_read: pmbus command READ_IOUT_CMD to CB 2 XF ASIC A failed
acb_pmbus_read: pmbus command READ_IOUT_CMD to CB 2 XF ASIC B failed

send: yellow alarm set, device CB 2, reason CB 2 PMBus Device Fail
send: yellow alarm clear, device CB 2, reason CB 2 PMBus Device Fail

alarmd[20914]: Alarm set: CB color=YELLOW, class=CHASSIS, reason=CB 2 PMBus Device Fail
craftd[19290]:  Minor alarm set, CB 2 PMBus Device Fail
alarmd[20914]: Alarm cleared: CB color=YELLOW, class=CHASSIS, reason=CB 2 PMBus Device Fail
craftd[19290]: Minor alarm cleared, CB 2 PMBus Device Fail

XXXXXXXXXXXX> show chassis hardware detail no-forwarding

Hardware inventory:
Item             Version  Part number  Serial number     Description
Chassis                                JN12373E3AFA      MX960
Midplane         REV 06   750-047853   ACRC4850          Enhanced MX960 Backplane
Fan Extender     REV 02   710-018051   CABV3056          Extended Cable Manager
FPM Board        REV 03   710-014974   ABCH8885          Front Panel Display
PDM              Rev 03   740-013110   QCS180950VW       Power Distribution Module
PEM 0            Rev 11   740-027760   QCS1728N16F       PS 4.1kW; 200-240V AC in
PEM 1            Rev 11   740-027760   QCS1816N0H1       PS 4.1kW; 200-240V AC in
PEM 2            Rev 11   740-027760   QCS1816N0FY       PS 4.1kW; 200-240V AC in
PEM 3            Rev 11   740-027760   QCS1816N02S       PS 4.1kW; 200-240V AC in
Routing Engine 0 REV 10   740-031116   9009207061        RE-S-1800x4
  ada0   3831 MB  UGB30SFA4000T2       SFA4000T2 00005   Compact Flash
  ada1  28496 MB  StorFly - VSFA18PI032G P1T120033369081 Disk 1
  usb0 (addr 1)  EHCI root HUB 0       Intel             uhub0
  usb0 (addr 2)  product 0x0020 32     vendor 0x8087     uhub1
  DIMM 0         VL31B5463F-F8SD DIE REV-0 PCB REV-0     MFR ID-ce80
  DIMM 1         VL31B5463F-F8SD DIE REV-0 PCB REV-0     MFR ID-ce80
  DIMM 2         VL31B5463F-F8SD DIE REV-0 PCB REV-0     MFR ID-ce80
  DIMM 3         VL31B5463F-F8SD DIE REV-0 PCB REV-0     MFR ID-ce80
Routing Engine 1 REV 10   740-031116   9009206700        RE-S-1800x4

Solution

This syslog message does not indicate an issue with the CBs . This is an intermittent minor error which indicates that one of the sensors within CBs was not quick enough to answer to the I2C request temporarily. This may occur when the RE is under stress.

This issue might be seen if the following conditions are met:
  • On MX platform
  • Routing Engine - "RE-S-2000 or RE-S-1800 is used"

This message can be safely ignored. Software improvement to handle this false positive has been implemented via PR1298612 - MX platforms may display false positive CB alarm "PMBus Device Fail" .

Modification History

2021-11-30: Added PR1298612

Related Information