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speed and duplex

hi all

if I hard code one side to 100m/full and the other side is auto, does it happen in many cases that the auto one will set itself to 100meg half ?

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Re: speed and duplex

Hello Carl,

the other side should always set itself as 100 half because without negotiation occurring the speed can be sensed but not the duplex

Hope to help



Re: speed and duplex

That is normal behavior and will "always" result in a speed/duplex mismatch . If you hardcode one side you have to hardcode the other, other leave both sides as auto , for the most part auto will work fine specially between cisco devices.


Re: speed and duplex

It also dependes and the devices network interface. We had some issue with Dell 520 not auto negotiating(auto/auto on both sides). It shows running full/100 but was getting a lot of CRC's. Once set to full/100 on both sides the crc's went away.


Re: speed and duplex

Setting the duplex mode on one side manually (to either full or half) and on the other side to Auto will always result that at the Auto side the duplex mode is half duplex. This because the auto-negotiation fails (as at the other side there is no peer with which to negotiate the duplex mode) and in such situation it fall-back to half-duplex.

This is a behavior specified in the standard.

As a result of that, setting manually to Full duplex at one side and to Auto at the other side will always result in duplex mismatch.

And to avoid this you have the choices:

- set auto at both sides (with some equipments auto-negotiation might not work corectly, so manual configuration is needed for those cases)

- set to full duplex at both sides

- set to half duplex at both sides (I listed this last option as well, even though this theoretical possibility should be never needed in reality)


Re: speed and duplex

One modification to the above posts should be noted.

If the auto-negotiated side is a Gigabit Ethernet interface, it will default to Full duplex. So, if you hard coded the other side to full duplex but left the Gig side as auto, you would not end up with a mismatch.


Re: speed and duplex

I forgot to add this exception, so thanks for highlighting this point.

As for GigaEthernet only auto or full duplex options exist (there's no half-duplex) then of course no fall-back to half-duplex will happen and so no mismatch will be created.

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