port flapping...

Unanswered Question
Apr 26th, 2010

We have been having network issues lately, and the only issues i see on our 4510R switch is with port flapping on a few ports....  my question to you is,   would

port flapping on a few ports cause the network slowness for everyone?  for example data transfer speeds going from 10 Mb per sec to 1 Mb per sec?......

Also, is it best practice to leave the autonegotiate enabled on Gb connections.... and not to force them to gigabit?.....  ..


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lamav Mon, 04/26/2010 - 08:05

It depends what these ports support. Are they trunk ports? What is connected to them?

Have you narrowed down the latency to a certain group of users? Or narrowed it down to a certain application?

These are the questions you have to ask yourself.



Edison Ortiz Mon, 04/26/2010 - 08:48

It is Best Practice to use AutoNegotiation on GigaPorts.

Port Flapping can affect latency if the ports are part of the transit flow.

It also causes STP to recalculate if running Layer2 services on those ports or a routing protocol to converge if running Layer3 services along with dynamic routing.

All these issues will affect switch performance and must be corrected. Can you post the log from the flaps and indicate what's the connected devices on these ports?



lamav Mon, 04/26/2010 - 08:54

You shadowing me, papi??

Where have you been, by the way? I was afraid you got sent to Arizona on a gig and ended up caught in the anti-Latino dragnet! lolol

Edison Ortiz Mon, 04/26/2010 - 10:12

The attached file is very hard to read. I'm looking for logs from the switch console while the flaps are being reported.

davisr651 Mon, 04/26/2010 - 13:48

lol...   sorry bout that the large file guys.. ... i will send another

lamav Mon, 04/26/2010 - 10:15


Im not abot to sift through 5000 lines of output that you sent.

Are you saying that gi3/2 is the flapping interface? If so, have you addressed it? Have you replaced the fiber or bounced the ports or used different ports?

You also say its connected to another switch, fine. But is it a trunk or routed connection? As Edison pointed out, the failure in each case will cause different issues.


shailesh.h Mon, 04/26/2010 - 10:56

I also faced similar issue of flapping the port and finally it was the problem of the VMWARE configuration (virtual interface) with same MAC address. In nutshell if problem limited one or two interfaces then check

  • What is connected other side
  • What is the configuration of flapping port (trunk / access)
  • Share the output of spanning tree and VTP status for switches
  • Try to capture logs from switch console if possible.. it give provide specific logs

As suggested by Victor, most of the answer have to answer by you and share the output. We will try to suggest something..

With regards,



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