Catalyst 3750-24P single port has negotiation problems

[ElQueue]

I have a Catalyst switch that no matter what cable or device I put on port 3 I get auto negotiation errors where one side will detect full duplex and the switch detects half duplex. If I put it into full duplex there's no problems. Any suggestions besides "don't use it"?

[luis_cordova]

Hi @ElQueue ,

Check this link to solve some configuration problem:

https://www.cisco.com/c/en/us/support/docs/lan-switching/ethernet/10561-3.html

I hope this can help you.

Regards

[ElQueue]

Yes I'm familiar with how it all works, and how to hard code it, I'm just confused as to why a single port out of all 24 of them has the issue with auto-negotiation, and if it's a symptom of a bigger issue.

It's happening regardless of whether it's connected to a 10/100/1000 PC, or another 3750 switch (both complain about duplex mismatch in debug), or anything for that matter. The switch always negotiates half duplex and the other end always negotiates full, even with different cables. Problem does not follow to other ports. Problems go away if I manually set both sides to full duplex on this port.

I'm not saying I can't work around it, simply that I want to know why it's happening.

[Leo Laohoo]

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@ElQueue wrote:

Yes I'm familiar with how it all works, and how to hard code it, I'm just confused as to why a single port out of all 24 of them has the issue with auto-negotiation, and if it's a symptom of a bigger issue.

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Depends on how OLD the IOS is.  

[ElQueue]

iOS 12.2(55)

[Leo Laohoo]

Depends on the IOS version.

[ElQueue]

12.2(55)

[paul driver]

Hello

does this occur with other ports?

whatvare connecting to the switchport?

have you tried defaulting the port ?

[ElQueue]

1. Does not occur on other ports except once on port 15 for a day. Made my EtherChannel bundle go nutty.

2. I've tried connecting a Windows 10 PC, an identical Catalyst 3750-24PS switch, and Cisco 1841 router. On port 3 it always detects half duplex, while the remote device always detects full duplex.

3. Yes, I've even cleared the configuration of the switch once.

[ElQueue]

On a weird note, I made all my ports on both my core and intermediate switch 100/full and now that makes the connected ports not even talk to each other. THEY HAVE TO BE auto/auto. WTH?

[Georg Pauwen]

Hello,

on  a side note, if you have one of the original v1 3750 switches, keep in mind that the end of life date is 2010. So the switch could easily be >10 years old, which by itself could cause this issue...