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Date:      Sun, 17 Oct 1999 12:07:40 -0400 (EDT)
From:      Garrett Wollman <wollman@khavrinen.lcs.mit.edu>
To:        jonathan michaels <jon@caamora.com.au>
Cc:        freebsd-net@FreeBSD.ORG
Subject:   ethernet switch type and freebsd  (Re: can two fast ethernet cards work in a freebsd box ?)
Message-ID:  <199910171607.MAA46454@khavrinen.lcs.mit.edu>
In-Reply-To: <19991017163640.B24189@caamora.com.au>
References:  <199910161104.TAA26753@eembox.ee.ncku.edu.tw> <Pine.BSF.4.10.9910161424540.81531-100000@mail.wolves.k12.mo.us> <19991017090323.A23931@caamora.com.au> <38094456.B210ECEC@softweyr.com> <19991017163640.B24189@caamora.com.au>

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<<On Sun, 17 Oct 1999 16:36:40 +1000, jonathan michaels <jon@caamora.com.au> said:

> i've heard a reasonable amount of chatter, when its been 
> raised, about switch "layer N".

This refers to layers in the new-irrelevant-but-still-popular OSI
Reference Model.  Layer 1 is physical, layer 2 is link, layer 3 is
network, and so on up to 7 (application).

In the Internet reference model, the layers are subnetwork, internet,
transport, and application -- this cuts out the useless OSI session
and presentation layers.  (Shows what happens when your network is
designed by smart people who actually build things as opposed to a
committee.)  Historically, a packet switch operating at the subnetwork
(OSI link) layer was called a ``bridge'', and one operating at the
internet (OSI network) layer was called a ``gateway'' or more recently
``router''.

About eight years ago, the term ``switch'' was revived, to refer to
bridge architectures in which packet forwarding was implemented in
hardware, analogous to ATM's cell switches, and more specifically such
designs where the internal interconnection fabric had enough capacity
to handle all bridge ports operating at full line rate simultaneously.
More recently, numerous vendors have developed hardware router
architectures with analogous behavior.  It thus became natural to
describe these routers as ``layer-3 switches'', as compared to
bridging ``layer-2 switches'' -- and in fact most of the former also
implement the latter.

Finally, a whole new evilness has been visited on unsuspecting
humanity by the appearance of so-called ``layer-4 switches''.
Depending on which vendor you talk to, this may mean different things
-- the classic example of what Debbie Deutsch of Lucent calls a
``marketechture''.  To some vendors, a ``layer-4 switch'' is simply a
layer-3 switch which can look at transport headers to provide Quality
of Service functionality or packet filtering.  To other vendors, a
``layer-4 switch'' is a specific kind of NAT kluge used for
load-balancing across Web servers.

All of these names principally serve the purpose of the vendors'
marketing departments.  I prefer to use terms like ``bridge'' and
``router'' if I am referring to a specific type of packet switch, and
reserve the term ``switch'' for the generic.  But then again, I'm
lucky enough to work in a place where I can call something a
``gateway'' and still be understood....

> specifically, is thier support in freebsd fro different switch 
> 'layers'

Yes and no.  FreeBSD can act as a router (layer 3) or as a bridge
(layer 2), but the machines on which FreeBSD runs on typically do not
have hardware forwarding support or a sufficiently beefy bus
architecture to back it up.  On the other hand, there are several
vendors who will sell you a switch which happens to run FreeBSD on its
management processor, like Juniper.

-GAWollman

--
Garrett A. Wollman   | O Siem / We are all family / O Siem / We're all the same
wollman@lcs.mit.edu  | O Siem / The fires of freedom 
Opinions not those of| Dance in the burning flame
MIT, LCS, CRS, or NSA|                     - Susan Aglukark and Chad Irschick


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