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Re: LAN N/w thruput, Basic Qs - Off-Topic

From: Thomas Fox <>
Date: 2005-12-24 01:45:17

Actually I've seen some performance issues with new HP servers (DL380 and DL580 series) and Cisco 2950 switches. For some reason, nailing both the NIC and the switch slowed performance. It was the goofiest thing I had ever seen. Setting both sides to autonegotiate brought the link up to 100/Full, and performance improved.

However, on the same switch (and I tried swapping ports), older Compaq DL380 and ProLiant 8500 servers required the links to be nailed for performance, otherwise they linked at 100/Half.

This is definitely a "your mileage may vary" message.


> Just to develop that at bit further, allowing your switches / NICs to
> autodetect /
> auto-negotiate can do a lot worse than that... I've personally seen
> situations where
> a 100Mbit NIC was "maxing out" at somewhere between 5 and 10 Mbit/s until
> autodect was disabled. Granted, that was a *very* old PC running Windows,
> but I am led to believe that this situation is surprisingly common.
> I am told by people who know much more than me (not that this says much)
> that it is a good practice to hard-code link speeds and duplex settings
> rather
> than letting your NIC try to guess. The way I see it, even if this
> advice is
> nonesense it can't really hurt -- as long as you get the settings
> right. ;-)
> As for actual network bandwidth, don't be fooled by "nominal" bit rates.
> 100Mbit/s (or whatever) is a "theoretical" maximum. Token based networks
> can often come close to this nominal maximum, but most other networks
> where collisions are possible (e.g., ethernet) are likely to peak somewhere
> between 60% and 80% of the nominal maximum.
> 'Course, I'm *far* from being a networking expert. It's been years
> since I've
> been allowed into a wiring closet (and I don't think *anybody* has ever
> allowed me into one voluntarily).
> Cheers,
> -- Mark
> Thomas Fox wrote:
> >Sriram,
> >
> >You are correct in your answer.
> >
> >Another tidbit to keep in mind is that only switches support full duplex.
> If
> >you use a hub (not sure how many still exist), you are forced to use half
> >duplex.
> >
> >Sometimes, when using a switch, your NIC may autodetect a 100Mb half duplex
> >connection. This is a huge bottleneck, as the NIC can only send or receive,
> >not both, at the same time. Therefore, always, always check your link.
> Don't
> >trust that autonegotiate will do what is best. You'll always want a full
> >duplex connection over a half duplex connection any day.
> >[...]
> >
> --

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Received on Sat Dec 24 2005 - 01:45:17 CST

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