Switch fastethernet ports flapping up and down

[ifabrizio]

Dear all,

I have on my cisco 3550 the following errors on the log:

Nov 24 10:06:07: %LINK-3-UPDOWN: Interface FastEthernet0/16, changed state to up

Nov 24 10:06:09: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/16, changed state to up

Nov 24 10:06:23: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/16, changed state to down

Nov 24 10:06:28: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/16, changed state to up

Nov 24 10:17:25: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/16, changed state to down

Nov 24 10:17:29: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/16, changed state to up

Nov 24 10:40:34: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/36, changed state to down

Nov 24 10:40:39: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/36, changed state to up

Nov 24 12:59:21: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/42, changed state to down

Nov 24 12:59:22: %LINK-3-UPDOWN: Interface FastEthernet0/42, changed state to down

Nov 24 12:59:24: %LINK-3-UPDOWN: Interface FastEthernet0/42, changed state to up

Nov 24 12:59:26: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/42, changed state to up

Nov 24 12:59:39: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/42, changed state to down

Nov 24 12:59:44: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/42, changed state to up

Nov 24 14:03:28: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/3, changed state to down

Nov 24 14:03:33: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/3, changed state to up

Nov 24 15:28:15: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/23, changed state to down

Nov 24 15:28:20: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/23, changed state to up

this error is on the log every minutes.

Could you help me to understand why?

the PC connected to the 3550 are always power on.

Thanks & Regards,

Igor.

[ankbhasi]

Hi Igor,

Can you please confirm if all the ports are behaving the same way or only few ports and also can you paste the "sh run" from the switch.

I believe some negotiation problem is causing links to flap? Can you confirm f you have hard coded the speed and duplex settings on switchport.

Ankur

[ifabrizio]

Hi Ankur,

Thanks for your help, yes all ports are behaving the same way.

I don't have hard coded the speed and the duplex.

I attach a txt file with the "sh runn".

Best regars,

Igor.

[ankbhasi]

Hey Friend,

I got the reason for your switchport flaps. Bydefault all ports on 3550 are in dynamic desirable mode which is creating a flap because dynamic desirable mode negoitates to form a trunk and because the device connected to this does not support trunk it is flapping.

Configure this and you will be good to go

interface FastEthernet0/1

switchport mode access

switchport access

Apply the config for all the ports which are connected to your end stations and it will be stable then.

HTH

Ankur

*Pls rate all helpfull post

[ifabrizio]

Hi Ankur,

I have done, now i am looking for error on the log, tommorow i write you the results.

Thank & Bye.

Igor.