If three switch connect each other like to triangle. One switch(A) is root, second switch(B) is root secondary, third switch(C) is not enabed STP. Second switch(B) will take BPDU from root switch(A) and send to second Switch(B). this BPDU is spanning-tree-(for-bridges)_00(01:80:c2:00:00:00)
When BB#2(B switch) takes this BPDU, How to do BB#2 operate? Drop or Accept?