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Re: Routing vs Switching (was Re: Another UUNET Explanation)
From: "Craig A. Huegen" <c-huegen () quadrunner com>
Date: Wed, 2 Jul 1997 00:34:48 -0700 (PDT)

On 2 Jul 1997, Sean M. Doran wrote:

==>> Layer 2 heals itself, without the knowledge of layer 3.  Layer 3
==>> experiences a latency increase between certain paths.
==>> 
==>> Sorry, had to do it.
==>> *duck*

I've seen a "latency increase" on the order of 12 seconds round-trip from
San Jose to Reston, VA before.  Most layer 3 protocols don't like 6 second
one-way trip time for a packet (almost 100x greater than normal). 
Granted, this was a rare case, and the particular design at the time
didn't have a large negative effect (other than the reston-sanjose link); 
had it been a hub-and-spoke-based network and reston were a hub, I would
rather let the L2 backups do their jobs than re-route me through
Bangladesh and provide > 2s round trip to a lot of my leaf sites.

==>(To augment one of my own questions, when you have two
==>routers in a full mesh (FR, ATM, whatever using VCs) and
==>the VC between N-1 and N-2 goes out of service, how do N-1
==>and N-2 decide what path(s) to use to talk to each other?
==>Feel free to consider a network using any commonly
==>available IGP and the iBGP hack.)

Depends on the engineering... =)

I've purposely built configurations which consisted of 20 subinterfaces in
order to make a fully meshed network appear as a large number of
point-to-point links in order to combat some of the problem mentioned by
Sean in the "anecdotes" list.  This was a re-engineer from a multipoint
network due to a provider whose switched network was a bit green yet (no,
I won't mention who it was, so don't ask).

I've long been a fan of keeping layer 2 as the transport and avoiding
"routing issues" in favor of letting layer 3 control them.  Layer 2
should provide the pipes, and layer 3 can do the transport. 

I can accept that costs make up a big portion of the decision to use L2
switching in a network.  Particularly, the difference between using a
chunk of a carrier's fiber in the form of a switched service vs dedicated
services plays a large part in the decisions made for connectivity.

/cah



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