Comment 3 for bug 924446

So the issue is not STP itself, but the forward delay set on the bridges created by libvirt. Libvirt on oneiric creates bridges with STP enabled but a FD of 0. Precise has STP enabled but a FD 15s. With STP enabled, guests could obtain an IP during PXE/DHCP on the first try with a FD minimum of 8. I'd be hesitant to turn STP off by default, but perhaps either lower/remove the default FD if its safe OR document in the server guide/elsewhere that PXE booting KVM guests requires a minimal FD. At this point I'm unsure whether the default FD change has happened in libvirt, bridge-utils or elsewhere (the previously linked bugzilla+commit mentions libvirt defaults to 0)

Another relevant bugzilla ticket: https://bugzilla.redhat.com/show_bug.cgi?id=533684