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Community Member

Duplex Error


I am having the below issue.

Server A and Server B both are connected with Switch1. Both the servers are running with 100 Mbps Full Duplex and both the switch ports are configured with Auto. But the server A connected port showin as 100 MB Half Duplex. Because of this I am getting slow connectivity and the interface Output error also increasing gradually.

Please help me to fix this issue.

Thanks in Advance.


Re: Duplex Error

HI, [Pls Rate if Helps]

Pls note: Server A & B runs on 100Mbps & Full Duplex Mode.

Switch port should also be in the same Speed & Duplex Mode. Configure the Interface of Switch as Duplex = Full & Speed = 100

If Duplex & Speed Mismatch occurs you will have slow connectivity over the network.

Pls Rate If Helps ! !

Best Regards,

Guru Prasad R

Hall of Fame Super Blue

Re: Duplex Error


The simplest solution to this is to hard code the switch port to 100full and hard code the server to 100full as well.

Try this if you can and let us know how you get on


Community Member

Re: Duplex Error

Hi Jon

Yes, you are correct. But the problem is the connectivity to this server was fine before this. Suddenly the connection speed gone down. When I chekced the switch I foudn its working on a-Half a-100. How it will happen automatically.

bjw Silver

Re: Duplex Error

I've seen this issue in the past. Having the Server NICs and Cisco Switchports set to Auto/Auto is desirable and if the Server NIC is working to standard, it should negotiate to it's best capability (i.e. 100/Full).

When Auto/Auto negeotiates to "half" duplex on the switch side, it typically means that either the Server NIC has it's speed/duplex hard set or that the NIC is not working correctly.

I might suggest that you contact the Server NIC manufacturer, or browse their tech-support web site and check out any known issues.

Hope this helps.



Re: Duplex Error

Bad cabling can also cause the downshift to slower speed or half duplex.

Most often it's a "split pair pins 3&6" because someone didn't follow the correct pair order (per EIA/TIA 568a or b).

The split pair causes significant crosstalk and the NIC and / or port will downshift to 10BASE-T ... occasionally 100Half ... in half duplex, since only one side is talking at a time, the split pair is less of an issue.



Community Member

Re: Duplex Error


If the Cable is bad then there should be RUNTS error right. But I didn't get any thing like that except Output and (late) Collesion errors.

is there any other possiblities..?

Re: Duplex Error


I have seen this many times.

In your case someone may have changed the nic speed/duplex on the server. This will cause your issue...the previous responders are correct. Fixing the speed/duplex will resolve this issue.

The simple rule is if you have a fixed speed/duplex on the server Nic then fix the speed/duplex at Switch port otherwise Auto will usually be fine.

Hope This Helps




Re: Duplex Error

Have you tried another cable? Output errors generally indicate congestion ... which can be a swamped device upstream, or a crappy cable killing the signal / dropping frames and boosting re-transmissions (reenforcing the congestion).

Late collisions are also frequently cabling related.

Give it a shot.

Good Luck


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