TCP checksum and device driver eri0

During a snoop session on a 280R with solaris 9 and network interface 
eri0, I noticed that the TCP checksum field of every outgoing packet was 
incorrect.
This happened also on a Netra 120 with solaris 8 and on the same machine 
with solaris 9 (obviusly with eri0).  I tried also with tcpdump, but the 
result was the same.
The same packet snooped on the network has the correct checksum value.
I believe that something happens between the snoop point on the machine 
and the network interface eri0 output.
I also tried on other machines:  on a Sun 450R with solaris 9 (but with 
hme0) the TCP checksum snooped locally is correct, also on a Sun 250 
with solaris 8 and hme0 is correct.
With this last result I believe that the responsible is the eri0 driver. 
It is true?
If I want to snoop locally *but* with the correct checksum value, how 
can I do?
Any suggestion?   

Regards
Giovanni Porcelli