Router types
OSPF routers are classified into the following types based on their positions in the AS:
Internal router—All interfaces on an internal router belong to one OSPF area.
ABR—Belongs to more than two areas, one of which must be the backbone area. ABR connects the backbone area to a non-backbone area. An ABR and the backbone area can be connected through a physical or logical link.
Backbone router—At least one interface of a backbone router must reside in the backbone area. All ABRs and internal routers in Area 0 are backbone routers.
ASBR—Exchanges routing information with another AS is an ASBR. An ASBR might not reside on the border of the AS. It can be an internal router or an ABR.
Figure 19: OSPF router types
