Setting up LAN with 2 WANs connected to different routers

Question

I have a network built with 3 Asus RT-AC68U routers:

USB LTE modem (192.168.1.1) 
   |
router 1, NAT, DHCP (192.168.0.1) 
   | 
router 2, AP (192.168.0.2) 
   |
router 3, AP (192.168.0.3)

This setup works, but has only one poor WAN connection.

Physically each router is placed on a different floor. Router 1 is placed at the highest floor just below the roof and its USB modem is connected to an external LTE antenna. All routers are connected by an ethernet cable.

Soon I'll be getting another connection (fiber) which is going to be my main connection. However, I'd like to keep my roof LTE connection as the backup connection, so if the fiber goes down, all stuff in my network should failover to the USB modem.

Unfortunately the fiber is going to come from the (under)ground level to my house and it would be a very tricky thing to connect it directly to the router 1 on the top floor. I have no direct ethernet cable from the ground floor (router 3) to the third floor and placing such a cable will be a lot of work. I know such cable would make it trivial to use the Dual-WAN feature of my routers. The other way round is also not possible, because the antenna cables can't be so long to reach to the ground floor.

Instead, is it possible to have physical setup like this:

   \/
   ||
USB LTE modem (192.168.1.1) 
   |
router 1, NAT (192.168.0.1) 
   | 
router 2, AP (192.168.0.2) 
   |
   |LAN
router 3, NAT, DHCP (192.168.0.3)
   |WAN
   |
fiber modem (192.168.2.1)

My idea is to set up:

a static route on routers 1 and 2 to route all traffic to 192.168.0.3
a conditional static route on router 3 to route all traffic to the other gateway 192.168.1.1 if the WAN connection on router 3 fails (not sure though if that wouldn't create a loop...)

Would it work? Or is the idea totally wrong? If achieving that with static routing would work, is it possible to automatically add/remove/change static routes on Asus WRT (or maybe Merlin?) depending on the state of WAN connection?

Initially I tried to use the Dual WAN feature for that, because it has WAN status detection and can fallback to another LAN port, but I had to give up - somehow setting up a LAN port as the secondary WAN makes it no longer work as a standard LAN port and after doing this I could no longer access local addresses from router 3 (funny, Internet connection did work). :(

Piotr Kolaczkowski · Accepted Answer · 2019-10-22T17:36:34.863

I haven't tried the other solution that recommends to use VLAN, because, although probably possible, it needs instaling custom firmware on my routers, and I wanted to try out something less invasive first.

I managed to do it, so I'm posting my solution, because maybe it would be useful to the others:

==============
USB LTE MODEM
secondary ISP
==============
   | 192.168.1.1
   |
   | 192.168.1.53
   | WAN 
====================
ROUTER A (roof)
AP, NAT
====================
   | LAN           | LAN
   | 192.168.0.3   | 192.168.0.3
   |               +------------------------------------+                                     
   |                                                    |
   |                                                    X (no cable here)
===========================
SWITCH / AP                                         
192.168.0.2                                              
===========================                             X (no cable here)
   |                                                    |
   |                   +--------------------------------+
   | 192.168.0.1       | 192.168.0.1                    |
   | LAN               | LAN                            |
================================                        |
ROUTER B (ground floor)                                 |
DHCP server                                             |
gateway announced: 192.168.0.1                          | 
AP, NAT, Dual WAN                                       |
================================                        |
 | WAN1 (primary)      | WAN2 (secondary)               |
 | 192.168.2.100       | 192.168.3.100                  |
 |                     |                                |
 |                     | 192.168.3.1                    |
 | 192.168.2.1         | LAN                            |
============       ===============                      |
MODEM               ROUTER C  NAT                       |
                   ===============                      |
primary ISP            | WAN                            |
=============          | static config                  |
                       | IP:   192.168.0.4              |
                       | Mask: 255.255.255.0            |
                       | Gateway: 192.168.0.3 (!!!)     |
                       |                                |
                       +--------------------------------+

The trick is to use another router with NAT to make a local virtual LAN 192.168.3.0/24 that sends packets back to my main LAN, but to the different gateway. This virtual LAN serves the role of the secondary WAN.

score 0 · Answer 2 · answered Oct 19 '19 at 10:10

It looks as if you are currently using much of your routers as switch. That would explain the fact that your R2 and R3 have only one IP address.

Probably, your set-up looks more like this:

USB LTE modem (192.168.1.1) 
   |
   | 192.168.1.100
router 1, NAT, DHCP 
   | (192.168.0.1) 
   | 
router 2, AP (192.168.0.2) 
   |
router 3, AP (192.168.0.3)

The difference is in R1. You have an outside and an inside IP address on a router.

I have no experience with your specific router, but I see that it supports loadbalancing over two WAN connections. You should look in the documentation to seen if one of those WAN connections can be on a VLAN. I am proceeding from my Cisco knowledge of what should be possible.

If so, your idea to use a VLAN to connect R1 and R3 is a good solution. However, you should plan this carefully. I will outline a possible solution for you.

The connections between the "routers" (actually the switch-part of the routers) will be a trunk. Use 101 as the base VLAN for the trunk. Use 201 for the WAN-VLAN and 301 for the user-VLAN.

VLAN 201 will have subnet 192.168.1.0/24 VLAN 301 will have subnet 192.168.0.0/24

You will put the USB LTE in VLAN 201.

Your router 3 should then be able to load-balance between the WAN-connection in the basement and the WAN connection on VLAN 201.

Make sure that your R1 DHCP server serves VLAN 301. Put all the WiFi and normal access ports on VLAN 301, on all "routers". Default gateway for all systems should be the R3 address on VLAN 301.

Setting up LAN with 2 WANs connected to different routers

2 Answers2