DECnet and TCP/IP between Ultrices

[Brain in Neutral]

In article <880 at uakari.primate.wisc.edu>, bin at primate.wisc.edu (Brain in Neutral) writes:
> VAX 8200, Ultrix 1.2
> VAXstation 2000, Ultrix 3.1 (but comments apply to 3.0 and 2.2 as well)
> 
> Whenever the VAXstation kernel is compiled with DECnet in it, it won't
> speak TCP/IP to the 8200.  (Actually, the silence is only partial; rwho
> traffic gets through from one to the other, but ping, rlogin, rsh, etc.,
> don't work.)  When the VS kernel is compiled w/o DECnet, the machines
> talk TCP/IP to each other with no problems.
> 
> Anyone know why this is?

Thanks to those who responded.  The problem now appears to be fixed.  I'll
describe the solution below in case it might help someone else.

DEC uses 16-bit addresses for DECnet addressing.  Ethernet addresses are
48 bits.  DEC allocated a range of ethernet addresses from Xerox (the
address "custodian" for ethernet) that all begin with the 32-bit constant
aa:00:04:00, and then puts the DECnet address in the last two bytes to form
a 48-bit ethernet address.  (I discovered this this very morning while reading
"Designing and Implementing Ethernet Networks, Bill Hancock, on the bus
coming to work.  Funny how these things happen.)

Of course, that synthetic address may not correspond to the physical ethernet
address of the interface, so when Ultrix boots, the DECnet initialization
(presumably /usr/bin/ncp in /etc/rc.local) changes the address to one of
those aa:00:04:00 things.

Unfortunately the 8200 still had the old (unaltered) ethernet address in its
arp cache and the two machines would not talk to each other.  This was
fixed by deleting the arp entry for the VAXstation on the 8200 with arp -d.

Interestingly, our MIPS M/120 figured out the new ethernet address right away
and had no problems.

Paul DuBois
dubois at primate.wisc.edu