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Re: Internet Backbone Index
From: "Dorian R. Kim" <dorian () blackrose org>
Date: Sun, 13 Jul 1997 17:36:06 -0400 (EDT)

On Sun, 13 Jul 1997, Dorian R. Kim wrote:

There are many valid reasons why you'd want to build a switched WAN backbone
as opposed to a routed one in the current environment. However, to argue that
a switched backbone has an inherent advantage over a routed one is at best
disingenuous, and at worst dishonest.

It's been pointed out to me that I've neglected to mention the effects of
buffering delay. Rather than go on, I've included an article by Scott Bradner
on this topic:

-dorian

 ---
 
 title: Religious conversions?
 
 It seems that we can not get rid of the routers vs bridges war of words,
 even though it is now routers vs switches.  "Switch when you can and route
 when you must" is a mantra that one often hears from switching salesdroids.
 They claim that switching is faster, simpler, cheaper and smells better
 than routing.   This is not a new ideological conflict, it goes back many
 years to a time when some companies were building nation-wide bridged
 DECnet and SNA networks and routing was some esoteric weird thing that
 those IP guys did in dark rooms.  It seemed to die down for a while as IP
 became mainstream but has now flared again with the advent of Ethernet
 switches. (Even if many Ethernet switches are just multi-port bridges with
 a marketing makeover.)  But I am more than a bit suspicious that much of
 the religious-like fervor comes less from architectural purity than from
 product family. 
 
 Part of the FUD in this issue involves misconceptions about the performance
 of routing.  This is perfectly exemplified by an article a few weeks ago in
 this paper that said that IP switching can avoid "time-consuming route
 table lookups."
 
 Far be it for me to bring actual facts to the discussion, they do so
 distort the perceptions, but in the Harvard Network Device Test Lab we
 measured the forwarding latency of a Cisco Systems Catalyst 5000, a level
 two Ethernet switch, at 66 microseconds.  We measured the forwarding
 latency of a Bay Networks Switch Node, a level three IP router, at 72
 microseconds.  Thus the "time-consuming route table lookups" took about 6
 microseconds additional time in the Switch Node.  Somehow I do not think
 that a difference of 6 microseconds in router forwarding latency is going
 to be all that noticeable in a system latency which, at best, will be in
 the tens of milliseconds, and will be closer to 100 milliseconds in many
 cases.  And just to be clear, this processing is pipelined so that these
 devices forward packets at wire rate, as many routers have been able to do
 for a number of years.  Note also that buffering delays can dwarf
 processing delays.  It takes 1.2 milliseconds to transmit one 1518 byte
 packet on a 10 Mbps Ethernet.  It does not matter if the device is a switch
 or a router, if that packet is in front of your packet, your packet will be
 delayed by 1.2 milliseconds.
 
 Latency and its impact is just one example of the arguments that are used
 against routers and I, as a strong proponent of routing, expect that I
 could counter most of them.  But that may not be required.   A number of
 the switch companies are starting to come out with "level 3 switches", most
 of which are just fast, cheap routers with marketing makeovers.  I am
 already starting to see the painful process of mental realignment going on
 in the sales forces of these companies.  After years of damning routing
 they now must start to praise the concept.  It is a fun conversion to
 watch.
 




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