I am facing STP issue in my network.

I configured STP protocol in two branches like Building A and Building B both are Cisco SG300 24 port switch.

I configured dual Link primary and secondary which is connected in GE25 and GE26 Since 1.5 years working normal. (GE25 is the ROOT port and GE26 is the alternate)

right now i am facing some issue like getting packet drops and flapping.

While i am checking the logs and States is continuously changing the states listening, forwarding and blocking and some time port status is getting down and Up

Any one help us to resolve the issue

enter image description here

  • In order for us to help you, you need to post a simple diagram and your device configurations.
    – Ron Trunk
    Commented Jan 3, 2017 at 12:13
  • Hi Ron, i can 't attach the diagram Commented Jan 3, 2017 at 12:24
  • Don't see it. Edit your post to include it.
    – Ron Trunk
    Commented Jan 3, 2017 at 12:27
  • 1
    If nothing has changed, it sounds like you have a cabling problem. Or the port is failing.
    – Ron Trunk
    Commented Jan 3, 2017 at 12:32
  • 2
    Every time the port goes from down to up, it will go through the listening/learning/forwarding states. That is normal. Check or replace your cables.
    – Ron Trunk
    Commented Jan 3, 2017 at 12:42

1 Answer 1


It seems that you are not having an STP issue, but a cable issue. Any time a non-portfast enabled port goes up or down, a STP change (TCN) is sent and the port will cycle through the spanning tree states you see. Try running show int <port> for both the interfaces on both switches to see if you have any errors such as CRC. It's possible you have a bad cable or Gbic.

Also, you may want to consider changing these links to an etherchannel, instead of using STP to manage the redundancy. Doing this will ensure a faster failover in the event of an issue and increase the link bandwidth because both ports will be fowarding instead of one in blocking.

  • I changed the cable and port also i configured the GE 24 th port but still the port status is goes up and down. I am totally confused. Commented Jan 5, 2017 at 9:16
  • In this scenario spanning-tree portfast should be enabled or not? Commented Jan 5, 2017 at 9:54

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.