I'm not sure if this is the issue, but the last time I set up a vm with kvm and libvirt, I had to do a bunch of tweaking of iptables to get layer 2 traffic forwarded correctly. Maybe whatever you did to set up DHCP for net C changed the iptables config for that interface.
I only mention this because it bit me and you didn't mention anything about iptables in your description. Good luck tracking this down, and let us know how it turns out. /* PLUG: http://plug.org, #utah on irc.freenode.net Unsubscribe: http://plug.org/mailman/options/plug Don't fear the penguin. */
