Let's say I have a Linux VM. Default route is the gateway to the top of rack switch for public internet and a public IP is bound on one virtual nic.
2nd interface is on a private network so the VM can be reached anywhere on the VPN. This is a management network where the gateway is on the other side of the data center.
A lot of stuff sits on the 10.0.0.0/8 that needs to reach this vm so a static route for the second interface points that /8 to that gateway on say 10.100.100.1
Now inside the same cabinet are devices sitting on 10.20.20.0/24.
If I didn't do anything, would hitting something on say 10.20.20.2 route traffic through gateway outside of the cab and back? I would think so as it sees the routing table and has no way of knowing.
If I want to optimize traffic so nothing is routed and traffic stays local to the cab, could I just add a third nic and give it an IP of say 10.20.20.3 and hitting .2 would arp / hit it directly through the switch in the cab?
If the 10.0.0.0/8 network route is the most specific one, it will be found and used first.
Merely looking at your routing table should answer your question. You should not need static routes at all in your scenario.