Static routes and multiple routers

Question

I have the following set up:

My office is located out of town and unfortunately I can't get fibre. Fortunately, I have a mate in town who can get fibre. He has pointed a p2p WIFI modem at my place. His set up:

Internet
|
Cable Modem
|
Router (WAN: DHCP client: some internet IP DMZ Host: 192.168.20.100)
Router (LAN: DHCP server: 192.168.20.1/24)
|
p2p wifi modem
|

my office:

|
p2p wifi modem
|
Ubiquiti USG router (WAN: DHCP client: 192.168.20.100/24 GW: 192.168.20.1)
Ubiquiti USG router (LAN: DHCP server: 192.168.1.1/24)
|
Cisco 3750 Switch Port on VLAN 1 (LAN: DHCP client: 192.168.1.100/24 GW: 192.168.1.1)
Cisco 3750 Switch Port on VLAN 2 (LAN: DHCP server: 172.27.71.1/24)
|
PC (LAN: DHCP client: 172.27.71.100/24 GW: 172.27.71.1)

Cisco 3750 Switch Port on VLAN 3 (LAN: DHCP server: 172.27.72.1/24)
|
Server (LAN: DHCP client: 172.27.72.100/24 GW: 172.27.72.1)

Now in order to get it working I configured IP routing and DHCP on the 3750. This allow the PC and server to communicate via the Cisco switch as it is their common gateway.

In order to get internet access to the PC and Server I had to add TWO static routes...?

In the Ubiquiti USG:

172.27.0.0/16 via 192.168.1.100

And in my friends router:

172.27.0.0/16 via 192.168.20.100

This seemed strange to me, so I have triple checked and both routes are required.

Now we are getting to the questions:

1. 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?

2. 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?

3. 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?

I know I could get it to work by defining all the VLANs on the USG and running a DHCP server for each and just running a trunk to the Cisco switch (i.e. router on a stick) but then all the inter-VLAN traffic between the vlans would have to go via the USG and that is significantly slower.

Answer 1

1. 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.

2. 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.

3. 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.