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What issues can occur when two routers are set to the same subnet (no cascading, both cable connected to the same switch)?

For instance:

 - LAN: 192.168.1.0/24
 - Router A: 192.168.1.254 - No DHCP, connects to WAN1
 - Router B: 192.168.1.253 - No DHCP, connects to WAN2

LAN clients/hosts can use only one of them as a gateway

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What issues can occur when two routers are set to the same subnet (no cascading, both cable connected to the same switch)?

None. Each client uses its default gateway. You could also add entries to their routing tables, making them use both gateways depending on the destination.

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The only real problems are the ones you create. All the hosts will be able to talk to each other, and the internet via their respective gateway.

Problems arise when you configure a router to send traffic to a host that does not use it as the gateway. For example, if there's a NAT rule from WAN2 to host1 (1.1). That will not work because the traffic will not go back through WAN2.

Similarly, DNS optimization may give sub-optimal results. If a DNS query uses WAN1, it may get an answer that is specific to that network. With some CDN caches, it may return an answer that only works for WAN1.

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  • Similarly, DNS optimization may give sub-optimal results. If a DNS query uses WAN1, it may get an answer that is specific to that network. With some CDN caches, it may return an answer that only works for WAN1. - How could this happen? If a host is public, wouldn't it be accessible from any route? What could happen is that the route is not "optimised" therefore, additional hops could be required from the non-optimised route. Or?
    – Riccardo
    Commented Oct 22, 2020 at 9:14
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    Because many ISPs limit access to their local CDN cache(s) to their own customers. The point of the cache is to reduce internet traffic. Allowing random internet users to access that cache defeats the purpose. (i.e. I didn't install a netflix cache to give netflix free hosting / bandwidth.)
    – Ricky
    Commented Oct 22, 2020 at 9:24
  • Ahh, I see... Well in my case could be a problem also with one router with multi WAN connections to multiple ISPs...
    – Riccardo
    Commented Oct 22, 2020 at 10:06

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