<traffic reveals that the Sun always padded packets shorter than that.
<But packets below (IIRC) 64 bytes, the MicroVAX simply never received.
<We had kernel source for the VAX's OS, and as far as I could determine,
<the DEQNA just wasn't doing anything with them.

The DEQNA was not a perfect beast and likely the rev level of the one you 
have could have been early too boot.  Considering the the last rev level
of that board was N3(!) and a lot of the loose ones floating around were 
of the C3 rev there could be problems.  For some VAX configurations 
LPS40 printserver for example would not be reliable with rev level 
below E4.  While theey arent as bad as some say it's not that great 
either. Sufficient to say if you have one use it, if you have a choice 
use a DELQA.  However, often there isn't a choice.

Allison