[libvirt-users] virtual network bug?

I have had a number of occasions lately where a virtual "NAT" network (such as default) has stopped working. I have attempted to stop and then restart the network from the "network details" but the only thing that seems to work is to restart all of the virtual guests which use that network. I don't believe that I should have to do that. Is there some command which will "fix" things? I have had this happen to me a number of times but I am currently uncertain of the cause. Anyone know what is going on? I am not going to bugzilla this until. I do know one way to cause the problem. Disable the network in a virtual machine. Then using virt-manager's connection details menu, stop that (default) network ... then start it up again. Everything sort-of looks ok but it is not. Go back to the virtual machine and re-enable the previously disable network. Nope ... cannot connect. Start another virtual machine which uses that network and it works fine. In the "real world", I can disable a network, then remove a cable, connect the cable back up, re-enable the network and everything works. BTW, NetworkManager is used in all cases. Any ideas? Gene