when does tcpdump capture link layer frame?

i was using tcpdump to capture some raw ip packets ( and save to a file
) for furthur processing. i found, on my home computer, what the
tcpdump got is the ethernet frame, but on the computer of my office, it
is ip datagram only. both computer was running linux and connected to
similar ethernet LANs. would anyone tell me why?

another question is, on my home computer, can i let the tcpdump getting
rig of ethernet headers and only capture the ip datagram for me?

thanks.

-
woody