While digging into logs (debug spanning-tree event has been enabled) in order to troubleshoot communication losses after topology changes. I was able to notice once I triggered a TC, one specific access switch (connecting devices that are actually suffering from communication loss) is outputting every 2 minutes (and for hours) the following lines:
029156: Apr 17 2020 09:29:18: STP sync: Send ALL port data to all other switches... 029157: Apr 17 2020 09:29:18: STP sync: NULL sync_data -- nothing to send 029235: Apr 17 2020 09:31:18: STP sync: Send ALL port data to all other switches... 029236: Apr 17 2020 09:31:18: STP sync: NULL sync_data -- nothing to send 029273: Apr 17 2020 09:33:18: STP sync: Send ALL port data to all other switches... 029274: Apr 17 2020 09:33:18: STP sync: NULL sync_data -- nothing to send ... and so on for hours
No other logs about RSTP are showing up (port changing states during sync process for example). This switch is having only one uplink (Point2Point) to its distri switch. Other ports are edges. The same level of log was triggered at the same time on the distri switches but no RSTP logs at all are showing in this distri switch. Since there is only one P2P on the access, I was expecting this Sync signals to be sent from the distri switch but it seems not.
Can you help me to understand what's going on here? This RSTP Sync received every 2 minutes is quite curious isn't it?
From Cisco forum, I found this topic https://community.cisco.com/t5/switching/stp-state-sync-events-multicast-flooding/td-p/1708358 where they eventually think about a 802.1W bug... how can I confirm this?
Note: both switches are running IOS 15.2(4)E6