The OpenBSD stack does not by default result in the inclusion of the bridge code. To include bridging functionality the CDL option CYGPKG_NET_BRIDGE must be enable . It is also possible to enable more than one concurrent bridge. The number of bridges active in a device is configured by the CDL option CYGNUM_NET_BRIDGES, which has a default value of 1.
The default behavior of a bridge is to operate without the spanning tree protocol. When devices are operated in this default mode it must be ensured the network topology is loop-free. Any loops will cause broadcast storms and general mayhem on the network. Including spanning tree code during the build process will allow the bridge to communicate with other bridges. They can then detect such loops and by disabling selected interfaces break the loop. To enable spanning tree enable the CDL option CYGPKG_NET_BRIDGE_STP_CODE.