Sg550xg stack is crashing, "discarding" if other switch is connected

[redgasgiant]

I have 3 sg550xg in a stack, with sg350x edge switches. The 350s are each connected by LAGs (2 members to each lag, ie xg1/3 and xg2/3) using bi-di FS.com SFPs. 

Plugging and unplugging the cisco 350s is normal, pings are ok, no timeouts, acts like you would expect. Those are lag 3 and lag 21 in the logs. po3 and po21.

But if I plug in my industrial BlackBox LIE1014a, (lag #15 or po15) with any combo of SFPs (although no cisco brand sfp, we aren't rich) we get a drop. And, if we use the BlackBox brand SFPs it will crash whatever SG550 unit it is plugged into, requiring a reboot. 

The STP status page says both my cisco 350 edge switches enter a "discarding" state. The BlackBox stays "forwarding". (screenshot below)

The RAM log says the po3 and po21 say "STP status blocking" when the the Blackbox is plugged in also. (screenshot below)

We have tried adjusting auto negotiation settings, not using LAGs, different combos of LACP, different SFPs. I havent yet played with STP settings, as I don't have much experience there. 

Thoughts?

[balaji.bandi]

Looks like you have an STP issue as per the Logs, how is your network switch connected to other devices?

[redgasgiant]

Hi, So at the moment I only have 3 devices plugged into the 550x. 

The 2 other cisco sg350s are connected at 10g to the 550xg with bi-di SFPs, LAGs with LACP. 

The BlackBox is 1g, bi-di SFP. We have tried LAG and no Lag. 

What STP settings should I look at using?

[redgasgiant · Accepted solution]

I found my issue. I had competing STP bridge priorities.

Once I set the "Core" to zero, the issues went away (except for the blackbox SFPs crashing the stack)