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I have an asus Gigabit Router and a gigabit NIC (Realtek PCIe GBE Family Controller) witch is running at 100Mbps rather than 1000Mbps (1.0 Gbps). When I use an old laptop on the same Ethernet cable(cat6) its running succesfully at 1.0 Gbps. Means there's no problem with the setup, router nor the cable. I tried changing the speed & duplex from auto negotiation to 1.0 Gbps but its still running at 100.0 Mbps. I also have the latest driver update for my NIC.

I think im missing something but what ? What could be the problem ?

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I have solved the issue by upgrading the driver. The Realtek driver downloads can be found here.

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While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. – DavidPostill Jun 7 '15 at 19:26

Can you verify the CAT6 cable used (are you sure it's not CAT5/CAT5E?) is wired the same each end (check the colours on the underside of the RJ45's). Basically a CAT6 cable is a CAT5 cable with additional shielding.

If you had one end wired as T568A and the other as T568B then what you have is actually a 'cross-over' cable and not a 'patch cable'/'drop lead'. Originally intended for connecting 2 PC's together without a hub/switch the mistake is easy to make when wiring patch cables/wall sockets.

Which model NIC Realtek PCIe GBE do you have?

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I have both side wired as T568B and when i use my old laptop it work fine ..so the cable may not be a problem ..I have RTL8167 driver version 7.65.1025.2012 – sammy Dec 13 '12 at 14:58
    
This is the driver package I would use (from Realtek site, totally untested). – HaydnWVN Dec 14 '12 at 16:29
    
Here and here are users with similar issues, but with onboard NICs. Try some of the recommendations. The main one being - power down for at least 15 mins before rebooting. – HaydnWVN Dec 14 '12 at 16:32
    
i tried and doesnt work for me – Sam Dec 17 '12 at 1:15
    
My next step would be to use a 'Linux live CD' to test the hardware. – HaydnWVN Dec 19 '12 at 8:52

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