I thought I would go ahead and remove the firewall between our core router and the DSL router. I made as much preparation as I could before 5:00 pm, the end of the business day.
At about 5:05 pm I swapped some network cables around.
And the Internet broke.
I'm not sure why it didn't work. At first, I thought maybe it was the VLAN I created on the core router for the DSL router port (to segregate the network traffic and cut down on broadcasts) and reverted that.
The DSL circuit was still down, and my phone started ringing.
I then thought that I needed a crossover cable, but I was getting a link light when using the existing cable.
And the DSL circuit was still down.
The routing changes I had to make looked good, but still, no DSL.
Clearing the ARP tables didn't help [but I just now realized, in writing this, that I only cleared the ARP table on the core router, and not on the DSL router—hmmmm].
So I had to revert everything back while trying to keep myself from getting sick.