From: NELSON@MC.BITNET (Carl Nelson, Computer Operator I)
Newsgroups: vmsnet.networks.desktop.pathworks
Subject: RE: Station Address and the Ethernet Packet
Date: 6 Nov 92 08:07:00 GMT
Content-transfer-encoding: 7BIT
Original_To:  BITNET%"lanworks@miamiu"
Original_cc:  NELSON

>>Which address is used in the ethernet header for the packet
>>going out on the wire:
>>
>>    - The hardware address hardcoded onto the ethernet
>>      card (e.g. 08:00:2B:25:D9:A6)?
>>    - Or, the station address whose last three number
>>      sets are calculated based on the DECnet address
>>      (e.g. AA-00-04-00-B9-23)
>>                     ^^^^^^^^ calculated
>>
>>            ^^^^^^^^based on IEEE vendor IDs in the MAC address.
>>
>>DEC Tech Support just told me that it is the calculated address
>>that goes into the packet, and that the other will not appear in
>>any of the packet headers.
>>
>>So this is a reality check--can this really be true?
>>--
>>Kathleen Applegate, Sr. Analyst      apple@drvax3.msfc.nasa.gov
>>Marshall Space Flight Center             Voice:  (205) 544-7656
>
>        Yes, DECnet Phase IV has worked that way since we added support
>for Ethernet and began shipping it in 1982.  Gee 10 years now.
>
>        The local station address is calculated and set by the DECnet
>node address. (the 16 bit binary ((area*1024)+number) byte swapped)
>The upper 4 bytes are a constant.
>
>        Dave Mitton
>        Networks and Communications
>        Digital Equipment Corp.
>
 
  Yes _and_ NO!  If you are using REMOTE BOOT, then your station will use
BOTH of the addresses, at different times. When the card requests a
downline-load, it uses the built-in address, mainly because the ROM on
the board has no way of knowing what the DECNET address is, or should
be. When the DECNET MOP process on the VAX receives this request, it
uses the DECNET database to determine the file to download to the
system. Once this file is loaded into memory (just below the end of your
conventional memory area), the ROM transferrs control to it. This code
makes a request for a virtual disk service whose name is based on the
built-in address. Once this connection is made, it makes this 'disk'
'look' like the A: drive, and loads the DOS (or whatever) bootstrap from
the virtual disk and transfers execution to it. (FYI, the drive is
'moved' to the first non-used drive letter later in STARTNET. This is
why all of the CONFIG.SYS file references leave off the drive letter.)
 
  It's only when you get somewhere within the \DECNET\STARTNET.BAT file
that your system changes over to the AA-00-04-00-xx-yy format. This is
why you have to start DECNET before any other protocols, as the address
that the workstation uses/responds will change at that point. This
applies even to a non-remote boot setup.
 
  Actually, now that I think about it, I believe that the DECNET address
is actually used when the LADDRV.SYS driver starts up, or possably just
before DOS is loaded, as there is a file used by the Data Link Layer
(DLL) to set the software parameters for the board. Hmmm, I'll have to
do some more research on this one. Time to break out the protocol
analyzer and Pathworks documentation.
 
  If you are managing more than just a few systems, or using more than a
few protocols, you really should get a protocol analyzer, if only so
that you can become familure with what's REALLY going on on your wire.
You'd be surprized at all the stuff floating around out there, even on a
"simple" network. I use Software AG's EtherPeek. It runs on a Macintosh,
and is very easy to use. Not as feature-gonzo as some other systems, but
good for general occasional use, and is reasonably priced.
 
Carl Q. Nelson
System Demigod (or Computer Operator I, if you must)
Mesa Community College
1833 W. Southern Ave.
Mesa, Azirona  85202
(602) 461-7490
Bitnet: NELSON@MC
InterNet: NELSON@MC.MARICOPA.EDU
Newsgroups: vmsnet.networks.desktop.pathworks,comp.sys.mac.comm