Unifi Spanning Tree Link Flap

Morning Y’all,

I have a strange issue with two of my Unifi US-48-500W switches. One switch is RSTP Root Bridge with a value of 4096 (lets call it switch A) the second one has the priority value of 8192 (lets call it switch B). Both switches are connected via 2 SFP+ DACs and show 10G. Also there is a third Unifi US-48-500W switch (Switch C) which is connected to either of the mentioned above switches. After the last reboot. The links between Switch A and switch B Flap with RSTP Discard Messages.
Anyone got an idea?

Both switches show via “show spanning-tree mst detailed 0” switch A MAC-Adress as Designated Root

Also there is a constant Topology change in progess as it seems even if I have only one interface between Switch A and Switch B enabled

Let me guess, this happened after you installed the 6.2.11 firmware for the switches? If so, I saw the same thing. I tried rebooting the switches, downgrading, redoing the 6.2.11 firmware nothing worked except time. After 30 min of all switches being on 6.2.11 things quieted down. What seemed to set it off for me was having some switches on 6.2.11 and some on 5.76.7. When all were on the same version then the flapping stopped.

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solved the problem in the meantime. I updated all switches to the latest firmware. Then reset all RSTP values to default. Rebooted all switches. After that set all RSTP Root Bridge values again. Reboot again.
Now they all run like they should do. Don´t know what triggered that behaviour but was wierd as hell.
The issue even occured regardless of the two 10Gb/s DACs Uplinks between Switch A and Switch B are in a LAG Group or separate.

Well little Update here.
It was not really an issue with RSTP. It was/is a problem with the spfs from my DACs cables. Note that those are genuien DAC cable from unifi.
Apperently the new firmware does not detect them correct and those ports flicker between 1G and 10G all the time.
Got in contact with Unifi over their forums. They supplied me with custom firmware to test.
Will report back later if the new firmware solves those problems.