Re: Single subnet

Steve Deering <deering@pescadero.stanford.edu> Sat, 19 May 1990 17:10 UTC

Received: from merit.edu by NRI.NRI.Reston.VA.US id aa06907; 19 May 90 13:10 EDT
Received: Sat, 19 May 90 12:09:15 EST from Pescadero.Stanford.EDU by merit.edu (5.59/1.6)
Received: by Pescadero.Stanford.EDU (5.59/25-eef) id AA28247; Sat, 19 May 90 10:09:01 PDT
Date: 19 May 1990 9:20-PDT
From: Steve Deering <deering@pescadero.stanford.edu>
Subject: Re: Single subnet
To: Dave Katz <katz@merit.edu>
Cc: fddi@merit.edu
Message-Id: <90/05/19 0920.681@pescadero.stanford.edu>
In-Reply-To: Dave Katz's message of Fri, 18 May 90 103312 EST
Status: O

>	Is there any reasonable way of having single-MAC stations on both
>	rings when using the single-subnet approach?  ...
>
>	[description of problem of finding hosts on the other ring]
>
>	...so the only logical choice is a dual-MAC router. ...
>
>	[suggested ARP scheme]

As I mentioned in previous messages, another solution is a MAC-layer
bridge between the rings.  This does not require any changes to ARP,
and it removes the requirement for IP routers to be dual-MACed.  It
also solves the problem of IP broadcasts/multicasts reaching both rings.

Yet another scheme would be to use a kind of MAC-bridge/IP-router
hybrid, which acts as a MAC-layer bridge between the two rings and
as an IP router to other subnets.  (The ring bridging function could
be buried in the FDDI driver of an IP router.)

(To ward off flames from Vernon Schryver, I should stress that I am not
*advocating* such schemes, just identifying them.  I still favor either
the two-IP-subnets scheme or the all-single-MAC-stations-on-one-ring
scheme.)

>	By the way, CLNP does not suffer from this problem, since routers
>	know a priori where all the hosts are.

Yes, except that knowledge obtained while the rings are wrapped is
ambiguous.  Will CLNP routers detect the wrap->thru transition and use
that to trigger a solicitation for ES Hellos?

Steve