Chapter 1 What’s New for FortiOS 5.2.1 : New features in FortiOS 5.2 Patch 1 : Add a new option for STP forwarding
  
Add a new option for STP forwarding
Due to STP forwarding problem in one-arm transparent mode firewall, a new option: replace nothing (rpl-nothing) has been added when configuring stpforward-mode.
Syntax
config system interface
edit wan1
set stpforward enable
set stpforward-mode rpl-nothing
next
end