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Switchport STP state changes continuously, what it means?

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Switchport STP state changes continuously, what it means?

Hi,

 

Uplink switchport in our MS120-48LP (SW2) change continuously status from  designated→root and root→designated.

 

we lost connectivity with SW2 for almost one day and suddenly it came up during last night and after two hours it stopped with the STP status changing.

.

This switch (SW2) is connected with fiber to SW1 and SW1 is the RSTP root. When we had lost connectivity with the switch we got high CRC error on the uplink port in SW2. SW3 is connected at the same way to SW1 and it works perfectly. 

 

What cause the continuously STP status changing in uplink switchport? Is it related to Fiber & SFP or there could be another reasons? 

Thank you advance.

 

Here is the topology:

 

2020-03-24_08h46_09.jpg

 

 

 

 

2 REPLIES 2
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Kind of a big deal

Re: Switchport STP state changes continuously, what it means?

Hey @Herixon. I would guess that it is related to that link. What I would think is happening is that the STP BPDU's are getting lost in transit on that link, so SW2 is actually transitioning to root because it stops hearing BPDU's from SW1. 

 

When SW2's port is designated it thinks it is the root. When its port is root then it agrees that SW1 is the root. 

 

If there's no other devices between those two switches then I would certainly look at the components of that link. SFP's, patch cables, the fibre run itself... If you have a tester that's a good place to start, Swapping the modules (one at a time to isolate one as the problem if it is) is another good option. 

Kind of a big deal

Re: Switchport STP state changes continuously, what it means?

You may have some kind of funny loop as well.  Have you made SW1 the root of the spanning tree network?  If not - do that before trying anything else.

 

https://documentation.meraki.com/MS/Port_and_VLAN_Configuration/Configuring_Spanning_Tree_on_Meraki_... 

 

EDIT: Oops just spotted that you said it is already the root.

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