Path: utzoo!attcan!uunet!samsung!munnari.oz.au!csc!pte900
From: pte900@csc.anu.oz (Peter Elford)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Can subnets be separated by another net?
Message-ID: <2349.2697b7b8@csc.anu.oz>
Date: 8 Jul 90 22:46:15 GMT
References: <2346.2696efa3@csc.anu.oz> <9007080605.AA00749@cincsac.arc.nasa.gov>
Organization: AARNet
Lines: 70

In article <9007080605.AA00749@cincsac.arc.nasa.gov>, medin@NSIPO.NASA.GOV ("Milo S. Medin", NASA ARC NSI Project Office) writes:
> 	 > Is it possible to route between subnets of a class B address when the
> 	 > subnets are separated by another network?
> 	 
> 	 The short answer is no.
> 
> Well, that's not quite right.  This depends on your routing protocol and how your
> routers actually forward packets.

.. diagram deleted ...

> 	 No. The reasons is that the Gateways will only advertise a route to 137.147
> 	.0.0
> 	 (not to a particular subnet of that network) over the 192.9.200.0 subnet th
> 	ey
> 	 are connected too.
> 	 
> Again, this depends on the routing protocol and routers.  It is certainly easy 
> enough to configure the OSPF protocol to make this work.  In fact, the NASA
> Science Internet network (built of Proteon p4200 routers) does exactly this.
> We have 2 NSI routers seperated by a class C network, and both routers can
> deal with parts of 128.161 on both "sides" of the class C net.  This is
> a consequence of variable length subnet support, and how OSPF areas 
> are configured.

We are still just exploding out of the stone age Internet wise down here, so
it was pretty safe to assume RIP and normal (ie. fixed length) subnet masks.

> 	 The only way it *might* work is to give the interfaces that connect the two
> 	 gateways a second IP address (in this case from another subnet of 137.147) 
> 	 and use a bit of static routing in the hosts. Two subnets (from different n
> 	et
> 	 numbers would then share the same physical network).
> 	 
> 	 cisco routers support secondary interface addresses but will never generate
> 	 an
> 	 IP packet with the secondary IP address - which is why you might have to us
> 	e
> 	 some static routing. I've done something like this at Macquarie University 
> 	to
> 	 support CSIRO's links into that campus, but at that site it's a case of two
> 	 networks on the same cable, not a partititioned subnets (which is illegal
> 	 according to the RFC),
> 
> This is a real kludge, and any hosts on the net in the middle may cause problems 
> because of improper handling of broadcast packets.  As I said, with the right 
> routing protocol, this situation can work. 

I wasn't recommending the kludge; just using it as an example of what the
problems associated with partitioning subnets (of the RFC950 flavour).
 
> Now, I wouldn't go off recommending this approach to people, but it certainly can
> be useful at times, especially during transitions, and for other reasons too.
>
> So, Murray, the answer to your question is yes, given you have OSPF routers 
> involved.  Otherwise, I think the kludges you need would be pretty ugly and
> not work well.

I think Milo is saying that unless you are keen enough to dive into OSPF 
then the answer is no !

> OSPF is brought to you by the IETF, and is documented in RFC 1131.  Expect to
> see a multivendor demo at InterOp this fall.  Ask for it by name, accept no
> substitutes!  

How many vendors do you expect to see Milo ? (genuine interest, no sarcasm)

Regards,
Peter Elford,
AARNet