Tracing LAPB and X.25 on /dev/lapb:

To trace LAPB frames and X.25 packets as they are sent or received by LAPB, enter:

hostname# x25trace -i /dev/lapb 

To trace X.25 packets as they are seen by the X25 driver, type one of the two following commands:

hostname# x25trace -i /dev/x25 x25
hostname# x25trace