Thanks for the detailed reply - I guess it does look a little PowerPointy. It's all one problem that I was trying to be super clear on, and obviously failed!
SO: one single problem. There is a Mac Pro in the basement running Retrospect Desktop, and another Mac Pro as the client upstairs. There is also literally a single cat6 cable connecting the two Macs (no router or switch). I specified DHCP with manual address, and set them on both Macs. I copied the public key generated by Retrospect into the Public Key folder where the client sits.
I also used the ipsave command described elsewhere to require the client to use the dedicated cable port of the client Mac Pro, and the Retrospect preferences of the server to use that Mac's dedicated cable port. Proactive or not, it "kind of" works, but it doesn't seem right. If forced, the client shows "Protected by Retrospect", but then reverts to "No network connection". So on the client end, I can't ever check anything.
If I use the "normal" network connections (connected through a router with dynamic DHCP IP addresses), then it actually works OK. But that's not what I want to do; I wanted to keep the backup computer off of the network - hence the dedicated cable. I just don't understand why that connection wouldn't stay active. Gotta be a setting I'm missing somewhere.
Thanks again for the response.