In OSPF, an Area Border Router (ABR) is a router that connects one or more OSPF areas to the main backbone area (Area 0). Its primary function is to advertise routing information, often in a summarized form, between the backbone and the non-backbone areas it is connected to. A Backbone Router is any router that has at least one interface in Area 0. Since all inter-area traffic must pass through the backbone, both ABRs and other backbone routers are integral to the exchange of routing information between different OSPF areas.

A. An Internal Router has all its interfaces within a single OSPF area and does not forward routing information to other areas.

D. An Autonomous System Boundary Router (ASBR) is responsible for injecting routes from external autonomous systems into the OSPF domain, not for exchanging routes between OSPF areas.

1. Juniper Networks TechLibrary. (2023). OSPF Areas Overview. "An area border router (ABR) is a router that is attached to more than one OSPF area... ABRs are used to connect the OSPF areas to the backbone area."

2. Juniper Networks TechLibrary. (2023). OSPF Router Types. "Backbone router—A router in the OSPF backbone (area 0)... Area border router (ABR)—A router that connects one or more areas to the backbone area."

3. Moy

J. (1998). RFC 2328: OSPF Version 2. IETF. Section 3.1

"The Autonomous System". Defines ABRs as routers attached to multiple areas and ASBRs as those exchanging routes with other ASes.