Sniffing PPPoE

I have a 60CX-ADSL-A with OS 5.2.4

When I run

diagnose  sniffer packet adsl 

I get:

....

5.364104 pppoe printer havn't been added to sniffer

5.370829 pppoe printer havn't been added to sniffer

5.371005 pppoe printer havn't been added to sniffer

...

 

 

Any clues how can I do a sniffing on that interface?

 

Thanks!