In the reference topology, only physical interfaces were initially active, so the routers would have chosen their highest active IPv4 addresses as Router IDs (R1: 10.1.1.14, R2: 10.1.1.9, R3: 10.1.1.13). To avoid relying on physical interfaces, a loopback interface can be configured with a /32 mask (host route) and used as the Router ID. For example, if R1 is assigned the loopback address 1.1.1.1, it will adopt that value as its Router ID, assuming no other ID is explicitly configured.
