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Re: Another UUNET Explanation
From: smd () clock org (Sean M. Doran)
Date: 01 Jul 1997 23:21:09 -0400

Alex Rubenstein <alex () nac net> writes:

Frame Relay allows yout he ability to psuedo-directly connection various
pop's together, and gives that clean appearance of a 'no-hop' back bone. 
Why route when you can switch? 

Please explain this last line to me.

Consider a flat network, where each router has a
connection (virtual or otherwise) to every other router in
the network.

An IP datagram arrives at one of the routers.  

What happens now?

A clumsy tech or a backhoe takes out a physical path
between two POPs, each of which contains several routers
that form part of this flat network.

What happens now?

A bug partitions the FR fabric, making a few VCs go away.

What happens now?

A new route is announced to one of the routers in the flat

What happens now?  (You may wish to consider it both as a
route carried in some sort of IGP and a route learned from

Now unflatten the network so there is a maximally
hierarchical network (the opposite of a flat network),
in which only routers topologically adjacent to each other
have any form of circuit-like connectivity.  Go back and
walk through my list of questions, paying particular
attention to the issues involved in changes in IP reachability.

Now, for even more fun, consider variances in delay thanks
to the speed of light in copper & fibre.  Consider what
happens when IP reachability changes as traffic is being
sent towards the changing destination(s).

Finally, pretend you work for a large network that combines FR
switches and routers and which is taking it in the shorts
for some reason, and fix the problem so that I can get
from home to cesium.clock.org without frequent long pauses.

        Sean. (hi Noel)

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