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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Internet Backbone Index
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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