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From: kwe@bu-it.bu.edu (Kent England)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: Network config questions
Message-ID: <76149@bu.edu.bu.edu>
Date: 5 Mar 91 15:45:28 GMT
References: <32870@boulder.Colorado.EDU>
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Reply-To: kwe@bu.edu
Organization: Boston University Information Technology
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> From: markat@cyrano.llnl.gov (Mark Tassinari)
> Subject: Network config questions
> 
>            Bld1  Bld2  Bld3
> 	     \    |   /
>               \   |  /
> 	     ---------   ---------   ---------   ---------
> 	    |  Node1  | |  Node2  | |  Node3  | |  Node4  |
> 	    |  cisco  | |  cisco  | |  cisco  | |  cisco  |
> 	     ---------   ---------   ---------   ---------
> 	          \          \           /          /
> 		     \        \         /        / 
> 	                \      \       /      /
>        		           \    \     /    /
> 			      \  \   /  /
> 	                     -------------
> 			    |     hub     |
> 			    |    cisco    |
> 			     -------------
> 
> 
> My big question is,
> what's the best way to address the node<->hub links?

	Walt Haas asked the right question: Why the hub cisco if you
have leaf node ciscos?

	I ask the complementary question: Why leaf node ciscos if you
have the hub node cisco?

	If what you want is to use the cBus as your high speed
backbone, I suggest you plug each interface of the hub cisco directly
onto each subnet in each building, using fiber optic ethernet.

	If you have more than 18 or so Ethernets you need more than
one AGS+ chassis and then you have to link the hub routers together
with something.  Perhaps FDDI in future.

	This approach does away with the need for two port subnets.
However, whenever it is that cisco supports OSPF then you will have
variable length subnet masks and link state routing, both big wins.
With variable length subnets, you don't run into the problem you
described with subnet mask conflicts.  Your hosts are not affected and
can still use a common subnet mask and you can conserve subnet space
for lots of small subnets.

	There are two ways to plug the hub cisco interfaces into each
subnet: you can use the ODS fiber optic transceiver cable extender
solution or you can use a pair of standard FOTs.

	The ODS solution requires an Ethernet cable to plug into on
the subnet side, which is an advantage if you don't have a repeater or
bridge to attach to.

	The "pair of FOTs" solution is best if what you are attaching
to is the AUI port of a multiport concentrator.  FOTs give you better
link level error detection than does the ODS solution which is
pretending to be an AUI cable.

	If anyone knows of another AUI cable extender vendor than ODS,
let me know and I'll mention them in future.  All things equal, I
prefer FOTs, but I invite contrary comments and experience.  We have
had some trouble with ODS units aging, but we don't have a lot of
experience yet with FOTs.  Cabletron is our preferred vendor today for
FOTs.

	--Kent