Comment # 3 on bug 908375 from Peter Maloney

BTW. I have read the manual, and have always found it silly that nearly nobody
has STP on by default, despite what it says:

man brctl
       brctl  stp  <bridge> <state> controls this bridge instance's
participation in the spanning tree protocol. If <state> is "on" or "yes" the
STP will be turned on, otherwise it will be turned off.  When turned off, the
bridge will not send or receive BPDUs, and will thus not participate in the
spanning tree protocol. If your bridge isn't the only  bridge  on he LAN, or if
there are loops in the LAN's topology, DO NOT turn this option off. If you turn
this option off, please know what you are doing.


So I have always turned STP on. Old versions (and other OSSes) don't have this
issue with STP.