Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!NSIPO.NASA.GOV!medin
From: medin@NSIPO.NASA.GOV ("Milo S. Medin", NASA ARC NSI Project Office)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Routing and Multiple Subnets on one net
Message-ID: <9003022104.AA00803@cincsac.arc.nasa.gov>
Date: 2 Mar 90 21:04:42 GMT
References: <25ed4b52.69c4@polyslo.CalPoly.EDU>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 30


Ralph, I think you are the victim of excessive layering.  The problem 
is that while routers are not to forward broadcasts, they determine
what is a broadcast by looking at the destination IP address, and
NOT the MAC level header.  So if you run with multiple subnets on
a physical cable, you will typically also have multiple broadcast
addresses as well.  If a router then recieves a broadcast packet 
for a destination IP address other than the broadcast address 
configured into it's interface, it will try and forward it!

Personally, I consider every router guilty of this in violation of RFC-1009.
Though several quite respected people disagree with me on this.  The
real fix to modify the internal data structures from the driver to the 
IP forwarder to tag the de-encapsulated (de-ecapsulated from the link 
level that is) IP packet with a pseudo-header that keeps the information
about whether or not it was recieved via a MAC level multicast (broadcast
is a specific case of multicasting), and NEVER forward it in this case.
I'm told this is being doing in 4.4 BSD, but I would encourage folks
to beat up their router vendors to do this as well.  It violates the
principle of maximum robustness to do otherwise...

It's silly to throw away good information you pick up at layer 2, and
then use a hueristic to try and get around this at layer 3.  You can
never forward things with both 0's and 1's destination addresses, or
net broadcasts, or subnet broadcasts, but all this trys to fix
the symptoms, and not the problem, which is throwing away very 
valuable level 2 info.

					Thanks,
					   Milo