Hello,
On recent fedora-18 NetworkManager started to set default STP for the bridges to on, this is a change in the default of distribution, which up until now was off, I opened a bug[1].
If you experiencing disconnect of hosts from network due to STP detection, a workaround is available.
Simply add: --- STP="off" ---
To the wired interfaces (eth0, em1...) before host deployment, this will be copied to the management bridge and will prevent NetworkManager to use its default.
Regards, Alon Bar-Lev.