Ok, I have had this issue for 2-3 years now. I setup two networks, an External and an Internal (domain) network. I would then run software, let’s use Exchange for this example, that I want to be accessible from the External Network (SMTP, POP), but also have some stuff only for the Internal Network (Outlook).

Now everything would work great until at some point (seemingly at random) the network would stop sending responses via the External Network and try routing them through the Internal Network causing the server to show up as un-responsive. It took until now (2-3 years later) to found out what was wrong and how to fix it correctly and now I feel really stupid.

Here were my settings in the past:

 

 

And here is the correct answer:

 

 

 

The issue here was that by having a Gateway on the Internal Network, the computer would assume that it would be externally routable by default or that the Gateway (or its Gateway) would make sure it got routed correctly. By removing the Internal Network’s Gateway, it would know that only what the subnet says it could reach would be reachable and not try to go ‘up the chain’.

So, if you are having any issues where the traffic switches networks when you don’t mean it to, or the route table (automatic metrics) changes on its own to reroute traffic, try this.