Let's say I have 3 ROS1 nodes all publishing and subscribing to the same topic. 2 of them are running on the same machine and the last on another, and both machines are connected via a router. Will the 2 nodes on the same machine communicate over lo for the least amount of latency?
Likely, but it's mostly up to the networking stack of the OS to determine what the "best" route is for packets to reach their destination.
If not, and if all 3 nodes are running on the same machine, but the ROS_IP is set to its router-provided IP address, will all 3 communicate over the router?
I personally don't know any IP stack which would send traffic across a remote host if the destination for that traffic is a local address.
In other words: the router is / should not be involved at all if the destination of a packet is a local address.
But it's not ROS which "notices" or "optimises" this. That's basic IP networking.
How does ROS optimize topic routes in multi-machine setups?
The direct answer here would be: it doesn't, as that's not one of its responsibilities.
It's the underlying OS and its network stack which determine what happens in this case.
