I have got the Cabletron E5020 working on the SE/30.  

Unfortunately I don't how to properly make the changes to support it.  
I just edited if_aereg.h and changed the values of CT_ to match this 
card.

Here is what I used:
ROM: 0x00030000
RAM: 0x00010000
REG: 0x00000000
NetBSD 1.5 has the RAM and REG values reversed (For a different card?)

I was able to ping several local hosts and download a few packages from 
ftp.netbsd.org with it.

Here is the full info about the card:
Cabletron E5020 DNI Ethernet card for SE/30(10Base2+AUI)
Model No. 9050207 (On sticker on port board)
P/N 9000209-06 SE/30 (Main board)
P/N 9000207-02 (Port board)

-- 
Shawn D'Alimonte   sdalimon@home.com