unreliable ports - 3750

Answered Question

Hello,

It seems that every other day a different port on my 3750 24 port switch stops functioning. I will remove the attached device and put it on another switch. I tested all the bad ports a few days after their devices were removed, and they seem to function again.

The ports are configured to automatically detect crossover cables and negotiate speed up to gigabit, but otherwise the switch is not configured.

Has anyone else experienced this behaviour? Could this be a prelude to the switch experiencing total failure? What are some trouble shooting techniques I may need to resolve this?

Thanks in advance for you help.

I have this problem too.
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Correct Answer by donlerche about 8 years 8 months ago

Hi,

Whilst having the ability to auto-detect everything appears good news, this is not always the case, and can lead to problems such as you are experiencing. When these auto detects go wrong they will lead to errors on the ports, and I suspect that what you may be experiencing is the port shutting itself down due to too many errors. You can check using "show interfaces status" command to see if the affected ports are being listed as being "err-disabled".

Recommendations : -

Only use crossover cables to connect switches to switches (or hubs). For switch to PC, switch to router etc., use straight cables. Don't rely on auto mdix to sort out if you use the wrong patch.

ALWAYS match the link duplex settings on the switch to the setting on the equipment at the other end, i.e. if the PC is set to auto, make sure the switch is auto, if the PC is hard set to full duplex then the switch MUST be hard set to full. The rule is NEVER set one end to auto, when the other has been hard set to half or full duplex. Note gigabit Ethernet only works full duplex.

Link speed settings are not normally so much of a problem, however always found it good practice where one end has been fixed at a particular speed then also fix speed at the other end to match.

Hope this helps

Don

Correct Answer by gpulos about 8 years 8 months ago

Quite a few things could be a cause but here are a few basics to start with:

1) cabling - shorts, interference, static/EMI, crosstalk: these could lead to so many errors that a port disables itself.

do a 'show interface f0/1' (or whatever port you had a failure on and check for in/out errors, queue/buffer errors, drops, etc.)

2) speed and duplexing problems - if you've not configured anything, you likely have Auto speed and duplex set on all ports.

Try to setup a system connection with 100 Full (or 10 Half if you have interference) on both the PC/server and switch, and note the results

3) If you have Gigabit cards in your PCs/Servers verify the cabling is adequate. Cat5 won't cut it and Cat5E will have problems if you have alot of EMI in the environment or other interferences.

Try a Cat6 manufactured cable for testing if this is the case.

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Correct Answer
gpulos Thu, 05/08/2008 - 14:41

Quite a few things could be a cause but here are a few basics to start with:

1) cabling - shorts, interference, static/EMI, crosstalk: these could lead to so many errors that a port disables itself.

do a 'show interface f0/1' (or whatever port you had a failure on and check for in/out errors, queue/buffer errors, drops, etc.)

2) speed and duplexing problems - if you've not configured anything, you likely have Auto speed and duplex set on all ports.

Try to setup a system connection with 100 Full (or 10 Half if you have interference) on both the PC/server and switch, and note the results

3) If you have Gigabit cards in your PCs/Servers verify the cabling is adequate. Cat5 won't cut it and Cat5E will have problems if you have alot of EMI in the environment or other interferences.

Try a Cat6 manufactured cable for testing if this is the case.

Correct Answer
donlerche Fri, 05/09/2008 - 09:52

Hi,

Whilst having the ability to auto-detect everything appears good news, this is not always the case, and can lead to problems such as you are experiencing. When these auto detects go wrong they will lead to errors on the ports, and I suspect that what you may be experiencing is the port shutting itself down due to too many errors. You can check using "show interfaces status" command to see if the affected ports are being listed as being "err-disabled".

Recommendations : -

Only use crossover cables to connect switches to switches (or hubs). For switch to PC, switch to router etc., use straight cables. Don't rely on auto mdix to sort out if you use the wrong patch.

ALWAYS match the link duplex settings on the switch to the setting on the equipment at the other end, i.e. if the PC is set to auto, make sure the switch is auto, if the PC is hard set to full duplex then the switch MUST be hard set to full. The rule is NEVER set one end to auto, when the other has been hard set to half or full duplex. Note gigabit Ethernet only works full duplex.

Link speed settings are not normally so much of a problem, however always found it good practice where one end has been fixed at a particular speed then also fix speed at the other end to match.

Hope this helps

Don

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