Change Linux firewall bridge STP to off, and stop it from fwding BPDUs.
Given that in an Emulab per-experiment firewall, there is only one switch port in the experiment that is in the default control net vlan (the firewalled nodes' ports are only in the per-experiment private control net vlan), there is no risk of a control net loop, so it is safe to turn off STP for the firewall's control net bridge. However, when STP is off, Linux then seems to forward BPDUs across the bridge (i.e. https://lists.linuxfoundation.org/pipermail/bridge/2007-April/005406.html), which we don't want. They intended it to support transparent bridges, but this is not a transparent bridge, and there is no risk of it causing a loop scenario.