Router Cascade - Connect to one LAN and another WAN

Installing and Using OpenWrt Network and Wireless Configuration
1

Hey there,

I already wrote a post at a german board and they told me that OpenWrt should be able to do the job.

My problem is a bit special I think. I do have internet about cable right now, but there are many downtimes and also packet loss happens quite often. I already terminate the contract, but it is still running till february next year.

Now I want to test normal DSL - this will probably be my choice in the future. So I need to connect two different ISP to my network.

This is the planned structure:

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I already read that the Archer C5/C7 (flashed it form C5 to C7) should be capable to do this. If I need another device I am also up for that I guess. Already looked for a Loadbalancer to have both at the same time, but that is rather expensive.

What do I need to do to access "10"-LAN from "20"-LAN and the internet should be provided over the new modem?

I already searched the board, but I might have the wrong wording for it. Already found WAN failovers to other routers and so on, but I think this case is a bit different?

Thank you in advance!

Greetings
Andy

2

I could not put two pictures in the post as new user, so here is the structure right now:

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I left out switches, devices and so on.

3

To achieve full connectivity between the subnets:

  • Use a different subnet for each router downstream interface.
  • On each router add routes to the other subnets via the IPs of respective routers.
  • On each downstream router:
    • Disable redundant NATs if any.
    • Allow to forward traffic in both upstream and downstream directions.