Yes this is doable with two routers, what I call a poor mans solution.
For those with the luxury of two ISP connections, (hopefully from different sources and thus having redundancy) the following is provided as a "low cost" method of sharing the connections and providing one LAN (an alternative to purchasing a dual or multi-wan router).
1. Decide on which Router is going to be your PRIMARY/Master Router - which will be used for routing functions including DHCP serving. Typically the more stable, firmware featured and higher throughput unit is chosen as the Primary Router.
You will need to note the Primary Routers' LANIP structure, including the DYNAMIC DHCP Pool it assigns, its own LANIP, and its subnet mask. One does not have to make any changes to this router unless, as per the example below, you wish to adjust its Dynamic DHCP pool range.
2. The other (Slave) router will maintain its Internet ISP connection as per normal. However, this router will NOT be DHCP serving and the following changes will need to be made:
a. Turn "DHCP OFF",
b. MANUALLY ASSIGN A LANIP WITHIN THE SUBNET OF THE Primary Router but outside the Primary Routers' DHCP pool.
c. MANUALLY ASSIGN SUBNET MASK TO MATCH Primary Router, and
d. RIP/UPNP should be turned off.
3. Physically connect each router together via ethernet, LAN port to LAN port
(single cable path not critical, ie can be through a switch/patch panel etc).
4. One now has both routers setup to provide internet connectivity to PCs attached to either router. How the PCs are setup (internet connection properties via the Operating System) will determine which router they will get their internet from. In other words, to which router they are physically 'cable-connected', does NOT determine the ISP being used (WAN input).
5. Setting up the PCs is required and may be accomplished as follows:
a. PRIMARY ROUTER - If you want your PCs to get their internet from this router, simply setup your PCs/Laptops etc to get their connection AUTOMATICALLY (as most NIC Cards are defaulted to do).
b. SECONDARY ROUTER - If you want your PCs to get their internet from the slave router, you will have to MANUALLY ASSIGN their associated NIC Cards (internet connection properties) with the necessary parameters as follows:
i. A LANIP within the subnet of the PRIMARY ROUTER but outside the Primary Routers' dynamic dhcp pool,
ii. A subnet mask to match that of the Rrimary Router, and
(i and ii to be on the same LAN)
iii. The GATEWAY will be the LANIP of the Secondary/Slaver router.
6. Now one should have all their PCs up and running and communicating with ISPs corresponding to the respective NIC Card user directed setup parameters (Local Area Connection Properties/Internet Protocol TCP/IP Properties). Most importantly, all PCs are on one big happy LAN.
****Be advised that ONE CAN ONLY FORWARD PORTS from the PRIMARY/Master router.
7. Depending upon the Routers' firmware capabilities, and assuming one is using separately sourced ISPs, either ISP can provide service for all PCs, in the event that the other ISP service is not available.
Two methods are most likely:
a. Automatic - Some routers (zyxel with zynos firmware) have the firmware option called "TRAFFIC REDIRECT" or something similar which allows the user to state a backup Gateway for the PCs to connect through.
b. Manual - Admin will have to enter in the Local Connection Properties/Internet Protocol Properties of non-internet capable PCs and make the necessary changes depending upon which Router is still getting ISP traffic.
Example of two WANS and two routers setup:
LAN IP 192.168.1.1
old Dynamic DHCP Pool 192.168.1.2 - 192.168.1.xx
subnet mask 255.255.255.0
Will need to change Dynamic DHCP pool
new DCHP Pool 192.168.1.33 - 192.168.1.xx (for example)
Subnet mask 255.255.255.0
old LAN IP 192.168.0.1 -or any other default
Will need to change secondary router LAN IP to be within subnet of primary.
new LAN IP 192.168.1.2 (note now outside DHCP pool of primary)
Subnet mask 255.255.255.0
PCs that User wants to connect to PRIMARY Router ISP:
Go to Local Area Connection Properties/Internet Protocol Properties (TCP/IP) and ensure the following is selected;
-Obtain IP Address Automatically,
-Obtain DNS Servers Automatically.
PCs that User wants to connect to SECONDARY Router ISP:
Go to Local Area Connection Properties/Internet Protocol Properties (TCP/IP) and input the following;
-Manually Assign Gateway as 192.168.1.2
-Manually Assign subnet mask 255.255.255.0
-Manualy Assign LANIP as follows:
PC C-192.168.1.5 etc etc...
Ain't nuthin but the blues! "Albert Collins".
Leave your troubles at the door! "Pepe Peregil" De Sevilla. Just Don't Wifi without WPA, "Yul Brenner"