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Community Member

Links flapping on 3750

Hello folks -

I have a Cisco 3750 Switch that has trunk connections to 4 other 3550's. Out of the 4 trunks, 3 of them are flapping. Every couple of minutes, I see the 3 links go down and come back up. As a result, users are complaining about network slowness and disconnects.

Other than it being a physical cabling problem, what else could cause such an issue? I cleared all the counters on the interfaces that are having the issue, and since then, I see input errors on two of them and no errors on one of them.

All the trunk ports are fiber GigE ports. The 3750 stack has two units in it.

Any troubleshooting tips would be appreciated!!

Hall of Fame Super Gold

Re: Links flapping on 3750

That is a cabling problem. Sometime, the patch cable connectors aren't good quality, not fully inserted, etc.


Re: Links flapping on 3750

I agree, most definitely a cabling issue.

Replace the fibre cables with brand new ones. The ends may have microscopic dust particles on them causing transmission issues.

Community Member

Re: Links flapping on 3750

Surprisingly I don't see any of the 3550's log the flapping messages. I have enabled logging at the debugging level, but I don't see the 3550's complain about anything. Howcome it's only the 3750 complaining about the flapping??

Hall of Fame Super Gold

Re: Links flapping on 3750

Link flaps aren't always bidirectional. For example, if a receiver detects loss of signal and the quickly reacquires signal, you've a link flap side only.

Are you using identical optical modules on both sides ?

Community Member

Re: Links flapping on 3750

I had a company come out and check the fiber cabling between the switches. They detected no faults in the cabling.

The links are still flapping. Possibly the SFP's on the 3750?? Or the 3750 itself??

Community Member

Re: Links flapping on 3750

Do you have the Switch Port Fast command enabled on the access ports?

What do the debug output look like? Do you mind posting the running config? Masking all your private info?

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