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I'm having an issue where some of my servers are not autonegotiating to the 1GB/s speed from their 1Gb NIC interfaces. These servers are Dell R610s running RHEL 5 wired to a Cisco switch. I have attempted to force the 1Gb speed from both the switch and server sides but have had no luck.

Running ethtool eth0 on the server provides shows that 1000baseT/Full is a "Supported link mode" and is part of the "Advertised link modes" as well.

I've tried to force the 1Gb interface by first running ethtool -s eth0 autoneg off and then ethtool -s eth0 speed 1000 duplex full. After running the second command I get the following errors:

Cannot set new settings: Invalid argument
not setting speed
not setting duplex

The previous commands will work if I try to force the speed to 100 instead of 1000. This has been puzzling me since we have other R610s which autonegotiate fine to the 1Gb speed.

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FYI: 1GigE specification only supports auto-negotiation. –  Steve-o Aug 28 '12 at 13:47

2 Answers 2

If your switch and your server supports Gigabit Ethernet, you probably need to check your cabling. Not all cables support Gigabit Speed. Also, you may have a bad/damaged cable/connector, etc.

For example, you will NOT get Gigabit speed if your cable has only 4 conductors as Gigiabit speed uses the 4 pairs/8 conductors.

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Gigabit Ethernet requires Category 5e ("enhanced") or Category 6 network cabling at ALL points - that means from the computer/server to the jack, the jack to the patch panel, and the patch panel to switch. You may need a Cat5e certified patch panel and/or Jack as well. I've heard some people dispute these. And on SHORT distances, you MAY be able to get away with the cable/panel/jack being only Cat5, but I wouldn't try to. I have personally seen auto-negotiate and link reliability issues on gig connections when the cable path is NOT Cat5e or better.

This is fairly easy to test by getting a gig switch (cheap is fine) and a Cat5e or better cable and connecting the two directly just to see if the link speeds come up as gigabit. (If your in-wall cabling is Cat5 - especially likely in older buildings - then you may have an expensive rewiring job ahead).

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