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From: mogul@Navajo (Jeff Mogul)
Newsgroups: fa.tcp-ip
Subject: Re: interesting loop
Message-ID: <8508301920.AA23992@UCB-VAX.ARPA>
Date: Fri, 30-Aug-85 14:10:00 EDT
Article-I.D.: UCB-VAX.8508301920.AA23992
Posted: Fri Aug 30 14:10:00 1985
Date-Received: Sat, 31-Aug-85 21:22:57 EDT
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Reply-To: tcp-ip@ucb-vax.berkeley.edu
Organization: The ARPA Internet
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    Obviously the simplest answer is that as long as the Applitek system
    is working, we turn off the Pyramid gateway code.  However we would
    like that code to be available as a backup should the Applitek system
    go down.  It begins to appear that we are going to have to check
    specifically for this sort of situation.  However in a complex network
    topology, it may not be entirely clear who one should and should not
    be willing to gateway for.  It is possible that the real moral is
    that "transparent" gateways and IP gateways may have trouble
    coexisting.

What is going on here is that the two gateways (transparent and IP)
are based on different models of the world; the transparent gateway
(bridge) assumes that it should act like a piece of wire, i.e. that
the cables connected to it should appear to be electrically connected.
In this model, forwarding the broadcasts (ARP messages) is not only
legal but required.

The IP gateway does not assume this; specifically, the "ARP hack"
assumes that the cables are NOT electrically connected.  In this
model, forwarding of undirected broadcasts is a no-no.

I believe the moral is that one should not mix metaphors.  If you
don't want to use your Pyramid as a full-time gateway, buy a
gateway box from someone.  Scrap the Applitek, or use it to
physically extend a subnet and pray that it doesn't fail.

Question of the day: is it possible to put two Appliteks (or
any other transparent bridges) in parallel, for reasons of
reliability, without getting into a loop?  If not, then you
shouldn't expect that putting a bridge in parallel with a
gateway should work any better.