Unlike Wednesday [1], I have all the time I want to remove the firewall between our core router and the DSL (Digital Subscriber Line) router.
It went much smoother if a bit longer than I expected. The big stumbling block wasn't cached ARP entries like I suspected, or even the wrong type of cable but the phyical port configurations. When one side (the DSL router) had a link light, but not the other side (our core router) it hit me that it might be the port settings. Our core router had the port pegged to 100Mbps, full duplex with no autonegotiation of port settings, and didn't care for the DSL router attempting to autonegotiate the settings. Once I reconfigured the port on the core router to autonegotiate, the link came up, and OSPF (Open Shortest Path First) worked flawlessly across all five routers running it.
So now all that remains is getting the T-1 install for XXXXXXXX and weighing the routes appropriately.