How to verify Spanning Tree status

[Ethan and Mia]

Hi all ,

I am newbie learning networking

I have more than 40 switches  using PVST and make core-switch as ROOT

So, how can I verify STP status to make sure no any "loop" on my network   guide me please.

because sometimes when rebooted some switchs  I will see "%PM-4-ERR_DISABLE: loopback error detected on Fa1/0/20, putting Fa1/0/20 in err-disable state "  on some uplink interface

thanks

[marce1000]

  Ref : https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/28943-170.html

                  >to make sure no any "loop" on my network   guide me please.

 In essence the question should never be needed if the Intranet-network is solid designed and SPT is setup correctly with

 a known root-bridge (e.g.) . That's how it is done !

 M.

[balaji.bandi]

make a small diagram  - see any switch has loop in the network, if that is your requirement, then you need to plan such a way alternative path will be blocked and maintained consistance.

[Martin L]

First of all, I would not use PVST, not with 40 switches, Use Rapid-PVST and have backup Root as well as Root in Core area.  That being said, STP or Rapid-STP will create a loop-free topology.  Any issues in L2 topology may happen if we are not following designing guidelines and not protect network from failures and errors. 

But as others have mentioned, you must have solid design of L2 topology and Root Sw placement is critical (as well as backup Root).  There are always a chance of loops especially with 40 switches which depends on how they are inter-connected.  There should never be a path from A to B that goes over 7 switches per Cisco recommendation.  So, again design of L2 network is critical.

"err-disable state" errors are not good and may indicate issues.  STP will protect network and should put those links in error-disable state, which is good thing since we do not want or need loops in the network.  However, you said that you see that error after switch reboot, which may not be STP related. "loopback error detected" could be because of port is damaged.  We need some more investigating to be done for this specific error.

Regards, ML
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[Ethan and Mia]

My first met  "err-disable" for   this case it happened when power surge many times .

All switches connected like triangle(redundant link) I had verify all switch by show stp root  all of them show correct root bridge.

May be "err-disable" came from power surge ?

[balaji.bandi]

May be "err-disable" came from power surge ?

what is the cause of the error disable of the port ? Do you have any stromcontrol configured on the ports ?

you can see the reason for error- disable reason. so you can find what caused the issue.

[Ethan and Mia]

Didn't have any storm control on ports , Should I do it  

Thanks

[marce1000]

                                               > Didn't have any storm control on ports , Should I do it  

 Not advisable before the problems are resolved may even be counter productive in this situation. You should also look into situations as , people finding a network cable on the ground thinking it's from their PC and plug into outlet , but it came from another outlet and they create  a loop  (happened on regular intervals at our company). Youthen see events as you observe (too). Much more likely cause then power-surge (for instance).

 M.

[balaji.bandi]

No some time i have seen high utilize links get err-disable.

we need more information on this and config, what is the reason of the err-disabled?

you can find the reason here :

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/69980-errdisable-recovery.html#:~:text=There%20are%20various%20reasons%20for,Port%20channel%20misconfiguration

[marce1000]

                              >May be "err-disable" came from power surge ?

 - Unlikely because not related to networking and stp-protocols . Beside other reply also configure a syslog-server on all your switches to capture logging at a central point. Then networking events become more manageable to analyze and observe 'event-structures'.  You may also want to do this for snmp-traps using some free snmp manager software , for instance.

 M.