As you can see, these are very simple, straightforward configurations. All
interfaces are assigned to Area 0. Another interesting fact about creating a single area is that there are no ABRs or ASBRs. It is possible to have an ASBR without having an ABR. If external routes are injected into the area, the router injecting them will be considered an ASBR. In order to activate an ABR, any interface on the router must be assigned to a different area.
It is also important to recognize that the neighbor discovery was auto-
matic in this single-area configuration. Now let's move on to an environment where sometimes neighbors must be configured manually.