EX 4200 - Got Tx underrun for Port:1

Hi guys,

I have EX 4200 , JUNOS 12.3R10.2, and strange message :

Jun 28 16:17:29 ex4200 chassism[1349]: cm_cheetah_ifd_read_port_intr_cause: Got Tx underrun for Port:1
Jun 28 16:17:35 ex4200 chassism[1349]: cm_cheetah_ifd_read_port_intr_cause: Got Tx underrun for Port:1
Jun 28 16:17:35 ex4200 /kernel: simulated intr
Jun 28 16:17:37 ex4200 chassism[1349]: cm_cheetah_ifd_read_port_intr_cause: Got Tx underrun for Port:1

can somebody explain what is this message ?

Thanks