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From: rnicovic@polyslo.CalPoly.EDU (Ralph Nicovich)
Newsgroups: comp.protocols.tcp-ip
Subject: Routing and Multiple Subnets on one net
Message-ID: <25ed4b52.69c4@polyslo.CalPoly.EDU>
Date: 1 Mar 90 16:18:26 GMT
Reply-To: rnicovic@polyslo.CalPoly.EDU (Ralph Nicovich)
Organization: Cal Poly State University -- San Luis Obispo
Lines: 37


Netters,

I find something strange on our network which seems to involve all
Routers on our network
It seems to be an incompatability between the routers and the addressing
on our network, and I thought you might have some insight or at least 
see if my cenario makes sence.
First, we are running multiple (2) subnets on the same physical (DLL)
network. There are a number of papers that claim this is leagal, but
perhaps their setup was diferent. In our case the two networks (subnets)
are 129.65.16.0 and 129.65.160.0, the wrinkle is that the mask on
16.0 is ff.ff.f0.00 and on 160.0 it is ff.ff.ff.00 . 
What happens is that a kinettics fastpath gateway on 160 sends out
a apple link broadcast. imediatly each of our routers retransmits the
same packet to the physical (DLL) address of the router that connects
16.0 to 160.0 . This router between the two subnets has two ethernet
interfaces both on the same cable.
It is my understanding that routers do not route broadcast packets.
In fact that is the benifit of routers over Data Link Bridges.
This is my guess of what happens. 
The Cisco and all the other routers are on network 16.0 with a mask
of ff.ff.f0.00. They see this broadcast packet from the kinettics
since at the DLL level it is a broadcast and they must look at it.
They then apply their network mask to the IP destination address
of 129.65.160.255 (which is the propper broadcast for 160.0 .
When the routers apply this mask they see 0's in the host field
and therfore do not recognise it as a broadcast at the IP level.
They then send it to the router between 16.0 and 160.0 since
they know that path and feel the packets should be routed.
Personaly I would think that any packet that is a broadcast
at the DLL level should not be automaticaly routed. Mabye this
is not the case.
Any Ideas ?
Ralph Nicovich
Cal Poly State University
Network Engineering