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I have an old Win Xp box with a 100 PRO/100 VE NIC and connecting to a Win 7 box with gigabit NIC via an ASUS gigabit router and peer to peer. I was getting 76% network utilisation on the XP machine using AUTO as the value for Link Speed and Duplex which is fine.

But I decided to see what would happen if i selected Full Duplex 100Mbps on the XP box. I thought at worst it would be as good as auto detect since the Win 7 box it's transferring to is faster. Instead network utilisation dropped to 0.12% (as in a 10th of a percent) and the transfers just timed out.

No biggie to reverse back to Auto but my question is why is full duplex such a dog? Im not "stuck" but just dont understand. There are only 2 machines here connecting through a router so i cant see how that would cause alot of collisions etc (if thats the problem).

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up vote 2 down vote accepted

In Gigabit ethernet, autonegotiation is a mandatory part of the spec.

I suspect on the old card, turning off "auto" turns off autonegotiation entirely. This will prevent the router from working out how to talk to it - it won't just fall back to 10Mbit like 100Mbit ethernet would.

If you put it back to "auto" it will be full duplex anyway.

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