- If the Ubiquiti USG is NATing the outbound traffic, then why do I need a static route in my friends router that refers to the 172.27.0.0/16 subnet, isn't this out of context?
The route is needed so both routers know where to direct traffic destined to the 172.27.0.0/16 subnet, which is not part of their subnet.
- The Ubiquiti USG performs bandwidth accounting, but it doesn't list the server or the phone in its client list, it only lists the Cisco switch. If the Cisco switch does not perform NAT then why is the traffic being attributed to the switch?
That's because your Cisco switch does layer 3 switching which means that your router sees its MAC address as its point of contact.
- What do I need to change in my setup to make the USG aware of the devices connected to the Cisco switch - while still allowing inter-Vlan communication to occur on the Cisco switch?
It is tempting to suppose that replacing or eliminating the switch would simplify your setup and solve this problem. However I can't be sure since you may have reasons to arrange your 3 subnets that way, and it wouldn't be very easy to configure it differently.