Path: utzoo!attcan!uunet!mcsun!ukc!dcl-cs!gdt!exspes
From: exspes@gdr.bath.ac.uk (P E Smee)
Newsgroups: comp.sys.dec
Subject: DECNET MAC address use query
Message-ID: <1990Nov8.125106.18326@gdr.bath.ac.uk>
Date: 8 Nov 90 12:51:06 GMT
Reply-To: P.Smee@bristol.ac.uk (Paul Smee)
Organization: University of Bristol c/o University of Bath
Lines: 34

Not really sure this is the best group for this, but it's the
best-looking I can find.  Any other groups I should try?

We have a question regarding DECNET use of MAC addresses, which has
become important as we are rewiring ourself into a bunch of local
ethers interconnected by clever managed bridges.  We can't determine
the answer experimentally, as we are not running DECNET at the moment,
but we do anticipate local use of it in the near future.

We are told that DEC, rather than assigning a single MAC address to a
given machine, assigns a group of MAC address to each box, so that the
DECNET protocol can use some of the 'address' bits to send small
control messages around.  (I find this distasteful, but apparently it's
'legal' as long as they stick to their assigned address range.)

Can anyone confirm that this is true?  Even more useful, if it is true
can anyone tell me how many, and which, bits of the MAC address in an
IEEE802.3 (Ethernet) packet are used by DEC in this way?  (Hex mask on
the 12-hex-digit MAC address format would be best to avoid ambiguity.)
Do they do this to both the source and destination addresses?  Or only
to one (and if so, which)?

Vaguely related issue, one of the switches available on our bridges is
one to tell it whether to forward or discard Ether packets whose source
and destination MAC addresses are the same (excluding broadcast
packets).  Is anyone aware of any protocol under which it would be
necessary or sensible to forward such packets?  We aren't, and offhand
it feels like if the source and destination are the same, then they are
by definition the same box on the same side of the bridge and so
shouldn't be passed around.  Any ideas?

-- 
Paul Smee, Computing Service, University of Bristol, Bristol BS8 1UD, UK
 P.Smee@bristol.ac.uk - ..!uunet!ukc!bsmail!p.smee - Tel +44 272 303132