Duplex settings on a 10/100/1000BASE-T interface

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Aug 28th, 2008
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I have a NetOptics ByPass switch with a 10/100/1000BASE-T interface connected to a GigabitEthernet switch port on a 6509 IOS switch. The interface on the NetOptics is set to 1000 and auto. On the switch, the port is set with the following commands:

speed 1000

duplex full

I asked the network admin to have the switch port changed to auto on the switch, but he says that it will not make any difference. Is that true? there is no difference between the above commands and having auto negotiation on for the switch port?


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SJessulat_2 Thu, 08/28/2008 - 22:26
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if you're lucky, you won't face any problems. But if you do not set the same modes on both ends of the connection, you may run into things like regularly error messages on the devices or trunks not being established between the devices.

If you have the chance to change the setting, do it. This may save you a lot of time in the future searching for the cause of a problem.



Rolf Fischer Fri, 08/29/2008 - 00:04
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Generally it is never a good idea to set one end of a link to auto and disable auto-negotiation on the other end (by setting full duplex in your case). The auto link is capable to detect the right speed but not the right duplex mode. For this reason the NIC will normally choose half duplex. Having hd on one end and fd on the other will always cause problems when larger amounts of data is transmitted.



Rolf Fischer Fri, 08/29/2008 - 00:11
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Addition/Correction to my first post:

With Gigabit Ethernet the algorithm works obviously a little bit different than with 10/100 and regarding to the Cisco-link the combination Auto<>FD will result in FD on both ends.

That means your colleague is right!


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