Interfaces on a SG500 goes up and down

[Bilal Hadd]

Dear Cisco users,

I have a annoying problem with two of SG500 switches. Some of the switch ports go automatically down and after two seconds the go up.

Please see below:

Some more info:
Switch 1 is Master - Switch 2 is Slave (

Both switches are stacked trough S1, STP Port-Fast is enabled, Port S1 mode trunk is turned on. All the computers are connected trough VLAN 1.

Who has some suggestions ? Many thanks in advance.

[Bilal Hadd]

Any suggestions ?

[pwwiddicombe]

Is there anything in the log on switch 2 to indicate what the issue might be? 

What cabling is there between the 2 switches; and could you be exceeding recommended cable lengths?

[Kryztian7]

In my case, the ports go down and couple seconds after they go up, then the ports go into STP forwarding status. There is nothing else on the logs prior to the ports going down. I am working remotely so I don't know if the recommended cable lengths was exceeded.

These is what I have tried so far:

• Update firmware to the latest version.
• Turned off PorFast.
• Enabled BPDU-Guard on all the access interfaces.
• Disabled Energy Enhanced Ethernet.
• Disabled MDIX.
• Disabled trunk negotiation
• Hard coded the interfaces as Full Duplex and Speed 1000.

Any suggestions?

Thank you in advance!

[pwwiddicombe]

While you check for cable length, check also that they are Cat 6 cables, since you are running Gig speed.

On interfaces, make sure BOTH ends of the link are set the same.  Auto one end, fixed the other doesn't work well (although that's normally a performance issue, and shouldn't cause ports to bounce).

For a test, you could set both to auto; then set one end to 10 half (since you're remote).  If the bouncing stops, you probably have a cable quality or length issue...

[Kryztian7]

Can you believe we are still having this issue with this switch? I just discovered that the same thing is happening to multiple of our SG300 switches. I have called Cisco and they have NO IDEA of what is going on. I believe this is a firmware issue because I've found other people on the web having the same problem. We have updated the firmware to the latest version and still nothing. The cable lengths are ok. I actually have a printer and a UPS NICs connected to the switch with 5 feet cables and both of those ports are flapping just like the other ones. I disabled STP globally on this switch but the problem continued. I configured MSTP on the whole network and the problem continues. It is hard for me to believe that there is something in the network causing this issue because the interfaces flap randomly at different times.

Any help with this from anybody will be greatly appreciated.

Thank you!

[Kryztian7]

I am experiencing the same problem but with an SG300. This is happening to almost all ports for what I can see. STP and Port-Fast is enabled. Only computers are connected to those ports.

Any ideas?

Thanks a lot!