This article explains why "Major SFB [X] Fault" might be seen on MX Series routers and what should be done to resolve the error.

The following alarm is generated in the situation:

show chassis alarms no-forwarding
2 alarms currently active

Alarm time        Class Description
2023-05-21 03:53:33 UTC Major SFB [X] Fault
2023-05-21 03:19:57 UTC Minor SFB [X] Not Online

The alarm is mostly seen along with with "Minor SFB [X] Not Online" alarm or "Minor Plane [X] not online" alarm.

The message log displays the following:

alarmd[47352]: Alarm set: SFB id=83886473, color=YELLOW, class=CHASSIS, reason=SFB [X] Not Online
craftd[16868]: Receive FX craftd set alarm message: color: 2 class: 100 object: 137 slot: 1 silent: 0 short_reason=SFB 1 Not Online long_reason=SFB 1 Not Online id=83886473 reason=83886080
craftd[16868]: Minor alarm set, SFB [X] Not Online

alarmd[47352]: Alarm set: SFB id=16777609, color=RED, class=CHASSIS, reason=SFB [X] Fault
craftd[16868]: Receive FX craftd set alarm message: color: 1 class: 100 object: 137 slot: 1 silent: 0 short_reason=SFB 1 Fault long_reason=SFB 1 Fault id=16777609 reason=16777216
craftd[16868]: Major alarm set, SFB [X] Fault

spmb0 master 2 entry 255 data 0 not valid. Err code (0) result 0x34400000 
spmb0 master 2 entry 255 data 1 not valid. Err code (0) result = 0x34400000 
spmb0 I2C accelerator failed to read result 2 
spmb0  CMD_READ Write failed for group 0x9 device 0x51 bytes 1 err 5   
spmb0 READ failed addr 51 data 0  
spmb0 doon_cbc_fpga_i2c_read_common: I2C read to grp 0x0009 dev 0x51 offset 0x00 failed, caller doon_cbc_fpga_i2c_read 
spmb0 cmty_vale_sib_ideeprom_read: Failed to read SIB1 ideeprom 
spmb0 cmty_sfb_get_assembly_id: Ideeprom Read failed for sfb slot 1 at attempt no 5 !! retry 
spmb0 cmty_sfb_get_assembly_id: Couldn't read IDEEPROM for SFB #1, status 1 after 5 attempts  
spmb0 cmty_sfb_state_check: Couldn't get assembly ID for SFB 1. Marking faulty

If above logs apart from alarm are seen repeatedly, It confirms to be a hardware failure most of the times.
 

Switch Fabric Boards (SFBs) are the data plane for the subsystems in the MX router chassis.

These logs and the alarm are seen because of hardware issue(transient or permanent) on SFB.

To resolve the issue :-

1. Offline/Online the board using below command :-
               > request chassis sfb offline slot <slot-number>
               > request chassis sfb online slot <slot-number>

2. If issue persist, physically reseat the specific SFB.
3. Try to swap the SFB to another slot and check if SFB is working.
4. If issue still persist, collect and provide the following logs to Support for further investigation.
               > show chassis sfb  
               > show chassis fabric plane | no-more
               > show chassis fabric plane extended | no-more
               > show chassis fabric plane-location | no-more

2023-07-21: Drafting article

2024-10-15 : Adding MX2020 to platform categories, requested by dherrera