This article help to troubleshoot a rare scenario where fxp0 is not detected on MX router and due to this the in-band access to the router is lost.

In a rare scenario, the fxp0 interface might not get detected on RE after reseat or some other maintenance.

{master}

root@Router> show interfaces fxp0 terse       

Jan 18 07:16:53

error: device fxp0 not found

{master}
root@Router> start shell 
Jan 18 07:39:07
root@Router:~ # ifconfig fxp0
ifconfig: device fxp0 not found

{master}
root@Router> show chassis routing-engine 
Jan 18 07:19:10
Routing Engine status:
  Slot 0:
    Current state                  Master
    Election priority              Master (default)
    Temperature                 29 degrees C / 84 degrees F
    CPU temperature             28 degrees C / 82 degrees F
    DRAM                      16330 MB (16384 MB installed)
    Memory utilization          23 percent
    5 sec CPU utilization:
      User                       2 percent
      Background                 0 percent
      Kernel                     4 percent
      Interrupt                  1 percent
      Idle                      93 percent
    1 min CPU utilization:
      User                       2 percent
      Background                 0 percent
      Kernel                     4 percent
      Interrupt                  1 percent
      Idle                      92 percent
    5 min CPU utilization:
      User                       2 percent
      Background                 0 percent
      Kernel                     4 percent
      Interrupt                  1 percent
      Idle                      93 percent
    15 min CPU utilization:
      User                       2 percent
      Background                 0 percent
      Kernel                     4 percent
      Interrupt                  1 percent
      Idle                      93 percent
    Model                          RE-S-1800x4
    Serial ID                      9013046552
    Start time                     2024-01-10 05:54:08 UTC
    Uptime                         8 days, 1 hour, 24 minutes, 57 seconds
    Last reboot reason             Router rebooted after a normal shutdown.
    Load averages:                 1 minute   5 minute  15 minute
                                       0.44       0.37       0.31
Routing Engine status:
  Slot 1:
    Current state                  Backup
    Election priority              Backup (default)
    Temperature                 29 degrees C / 84 degrees F
    CPU temperature             29 degrees C / 84 degrees F
    DRAM                      16330 MB (16384 MB installed)
    Memory utilization          19 percent
    5 sec CPU utilization:
      User                       1 percent
      Background                 0 percent
      Kernel                     1 percent
      Interrupt                  0 percent
      Idle                      98 percent
    Model                          RE-S-1800x4
    Serial ID                      9009119479
    Start time                     2023-06-27 06:20:55 UTC
    Uptime                         205 days, 58 minutes, 8 seconds
    Last reboot reason             Router rebooted after a normal shutdown.
    Load averages:                 1 minute   5 minute  15 minute
                                       0.31       0.24       0.18

This error is reported due to a transient hardware issue with the RE or genuine HW fault with the management interface on the routing engine.

As a workaround to restore in-band access to the router, user can switchover RE mastership to other routing-engine.
 

To resolve the issue, try reseating the RE and check if the fxp0 interface is getting detected after the reseat.
If the issue is still present after physical reseat of the routing-engine then the RE needs to be replaced.

Please open a case with JTAC Support for troubleshooting further.