Architecture related - if you have advanced network setup - where router VMs use 2 NICs for 2 networks - in many corp environments - dual homed NICs on different networks for System VMs, circumvent firewall and other compliance policies, making not compliant if you are subjected to strict compliance... this is probably where openstack shines (not mentioning other issues)
Assuming end user can punch firewalls for system VMs to talk to CloudStack and VMs underneath.. whats is the possibility of us creating another type of "advanced network" - that is compliant with regulations and uses only 1 NIC for SystemVMs?
