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Topological significance of transport technologies [Was: Re: Another UUNET Explanation]
From: Paul Ferguson <pferguso () cisco com>
Date: Tue, 01 Jul 1997 20:20:28 -0400

At 05:00 PM 07/01/97 -0700, Robert Bowman wrote:

layer 2 vs. layer 3

tis like comparing a motorcyle to an automobile--both get you places
but in fairly different ways

I shouldn't be contributing to this thread, but what the hell.

Its not really Layer 2 vs. Layer 3, its how to integrate the
two layers and make it work. Mike O'Dell is fond of saying,
"Pure Layer 3 routed networks are dead," and I can understand
his point, although I don't necessarily agree with it. I do
understand, and I think its important for everyone else
to understand the point here.

Yes, they both get you there, but the pertinent summary to be
drawn from this comparison is that 'you' are the IP packet,
and you really don't care what the mode of transport is (e.g.
frame-relay, leased point-to-point lines, ATM). Each provide
a pipe. Some have more intrinsic flexibility than others (e.g.
virtual circuits) and therefore represent a significant reason
to employ a specific technology over another, given pricing, and
geographic availability.

Again, IP packets don't really care if it's a motorcycle, an
airplane, or an automobile (unless its a Harley :-).

It should also be noted that some technologies, such as
frame-relay are used only in *topologically significant*
places, ie. customer aggregation, for precisely these
reasons. In some networks, frame-relay is used for
customer aggregation, fast-ethernet is used in the PoP,
and ATM is used in the wide-area (just an example).

- paul

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