Tracing LLC2 and X.25 on /dev/llc2:

hostname# x25trace -i /dev/llc2 

The command above captures all LLC frames, including Unnumbered Information frames used to carry CLNP and ES-IS PDUs. When tracing at the X.25 level on an LLC2 link, MAC addresses are displayed as 0:0:0:0:0:0. Tracing at the LLC2 level on the same link returns the correct MAC addresses.