In order for you to have a fail over, lets say Site 1 were you want the Fail over to be in place has 2 WANS you are allowing in. So Site 1 Router, will need some setup. Lets say we are using a new Router 4011 with 10 ports.
So Site 1 has an IP Address with the Network 192.168.254.0/24
Site 2 Router has an IP address with the Network 172.16.1.0/24 its ISP That comes in isn't important because you will be setting up Router 1 with Router 2 detail information on how to get out, router 2 already knows how to get to its WAN and out, as long as it has been setup properly
You will be pulling the Second Fail Over with Router 2, there really isn't anything you need to do with Router 2 because its bringing in the extra source of internet feed if WAN1 goes down. So weather you are using a cable connection or Wireless Communication, these are the steps you will need to follow.
Step 1: Determine the Ports you are going to use, by default ether1 is off the bridge and is the designated WAN1 and we want to use ether10 as the second WAN2.
Step 2: Access the Bridge, click Bridge, > Click the Ports Tab > Remove ether10 from the bridge!
Step 3: Name your ports Ether 1 to WAN1 and Ether10 to WAN2. Click Interfaces: Double click on Ether1 and Rename to WAN1 and Double click ether10 and rename to WAN2.
Step 4: Now here comes the more interesting part. Lets say WAN1 has the IP Address 192.168.254.50/24 its Gateway: 192.168.254.1 /24 Its Network 192.168.254.0 and Subnet mask is /24 255.255.255.0> WAN2 is 172.16.1.10/24 GW 172.16.1.1 Network 172.16.1.0 Mask 255.255.255.0
> We need to set these IP addresses to the fixed Interfaces we removed from the bridge, WAN1 and WAN2
> Click IP > Addresses >
Click the "+" to add a new Address.
Type in for the IP Address: 192.168.254.50/24 and select the Interface WAN1
Click the "+" again>
Type in IP Address 172.16.1.10/24 and select the Interface WAN2
Now we need some NAT Rules! Because we have 2 different Subclass Address going on here>
Click IP > Firewall > Click the NAT TAB >
Here you should see a default masquerade rule,
You will want to make sure that this NAT rule is setup as the following, so click the default rule and open it.
Out. Interface: WAN1
Copy this rule or add a new rule and do the same setup but, in Out. Interface in general tab, change this to WAN2:
Next you will need to set the following IP Route Rules:
Click IP > Click Routes>
Click the "+" and add a new route:
Click in the Gateway Box: Type 192.168.254.1
Click in the Check Gateway Box: Ping
Click the Distance Box: Type in 1
Click the "+" Again to add a new Route:
Click in the Gateway Box: 172.16.1.1
Click in the Check Gateway Box: Ping
Click the Distance Box: type 2
Note: The Route with the lowest Distance number will be its priority the router will use to default all traffic out through
Now you can setup Mangle Rules within the Firewall, to allow for Load Balancing and Fail over to work, but the Fail Over really works with the IP Routes, Mangle Rules will allow you to use it for Load Balancing as my understanding goes.
I hope this helps and works out for you all.
Btw if you are doing this with Wireless Setup. Say you have a Wireless Radio and you are feeding the Second WAN2 to Router 1, you will want to setup the Wireless Radios with IP addresses you excluded from Router 2. so for instances,
Router 2 site: has the Access Point and you set that up with an IP Address 172.16.1.7 /24
Router 1 Site: Has the Bridge Radio you will set this up with IP address 172.16.1.8/24 and plug that into Ether10 (WAN2)
Remember that you need to setup the IP address for WAN2 with a different IP address for Router 2 to be used, so the IP address configuration Should be for WAN2 172.16.1.10/24
I hope this helps and isn't to confusing.