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From: matt@ODDJOB.UCHICAGO.EDU.UUCP
Newsgroups: mod.protocols.tcp-ip
Subject: Re: (none)
Message-ID: <8703201752.AA18400@oddjob.uchicago.edu>
Date: Fri, 20-Mar-87 12:52:39 EST
Article-I.D.: oddjob.8703201752.AA18400
Posted: Fri Mar 20 12:52:39 1987
Date-Received: Sat, 28-Mar-87 06:21:43 EST
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Reply-To: matt@oddjob.uchicago.edu (Matt Crawford)
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jerry@oliveb.ATC.OLIVETTI.COM (Jerry F Aguirre) writes:
)                         ...  Ignoring the Bridge product, suppose that
) two departments each set up independent ethernets.  They properly
) register their IP addresses so there is no conflict.  Later they decide
) to attach the two segments together either directly or thru a repeater.
) ...
) Would it be enough to specify routing to the network via itself?
) Would something like:
) 
) 	route add net othernet myhostname
) 
) be enough to tell it that "othernet" is actually available via the same
) interface?  Someone out there must have had to connect two previously
) independent ethernet segments.

You bet someone has.  As I told Charles in private ail, the
"USAN" satellite network links many networks around the country
using Bridge/Vitalink filtering forwarders.

It does suffice in 4.2 and 4.3 to do

	route add othernet myname 0

where the final number, commonly believed to be a hopcount, is
actually for the flags field.  Zero means "not a route to a
gateway".

Trouble comes, however, if there is a host on othernet that you
want to use as a gateway to some more distant destination.  The
kernel cannot be convinced that the gateway is reachable, even
though it can communicate with the gateway.  If, as Charles
pointed out in his replies to me, the gateway will do
promiscuous arp, or if some other host on the net will do proxy
arp for the gateways, the day is saved and you can make a
default route to your own interface with

	route add default myname 0

An alternative which I used in the early days of USAN was to add
a pseudo-interface driver for each net reachable by bridges.
This driver just put its output packets on the queue of the real
eternet interface, while serving as a marker to the routing
ioctl code that gateways on those nets were indeed reachable.

Because of the net hassle of trying to solve this problem all
around the country, the final answer wound up being to connect
each bridge to a stub ethernet that linked it only with an IP
level forwarder.
________________________________________________________
Matt	     University		matt@oddjob.uchicago.edu
Crawford     of Chicago     {astrovax,ihnp4}!oddjob!matt