Ok so hopefully this is a quick answer. All of the explanations on the web I have found have been unspecific in this regard, so I apologize in advance for simplicity.
In the example topology, when configuring the MC and specifying the information of the BR (R3), whose interfaces are you specifying?
For example config:
<R5>oer master <R5>border X.X.X.X (Loopback on R3 or whatever) <R5>interface (INT) internal <R5>interface fa0/0 external <R5>interface s1/2 external
For those two interface commands, do I have that correct? Are we specifying the interfaces on the BR or the MC? I feel like it should be the interfaces of the BR being specified on the MC, but at the same time I'm not entirely sure how the MC would understand what those interfaces are being referenced.