Which OSPF router type is responsible for connecting OSPF areas to the backbone?

An Area Border Router (ABR) is a fundamental component in a multi-area OSPF network design. By definition, an ABR is a router that has interfaces in at least two different OSPF areas, and one of those areas must be the backbone area (Area 0). Its primary role is to connect a standard, non-backbone area to the backbone, summarizing routing information from its attached areas and injecting it into the backbone as Type 3 LSAs (Summary LSAs). This hierarchical design helps to limit the scope of LSA flooding and reduce the size of routing tables on internal routers.

A. Backbone Router: This is a general term for any router with at least one interface in Area 0. An ABR is a type of backbone router, but this option is not specific enough.

B. Autonomous System Boundary Router (ASBR): An ASBR connects the OSPF autonomous system to an external routing domain (e.g., BGP, EIGRP), not one OSPF area to another.

D. Internal Router: An internal router has all of its interfaces within a single OSPF area and does not connect to other areas.

1. Moy

J. (1998). RFC 2328: OSPF Version 2. The Internet Society. Section 3.1

"OSPF router types". It defines an Area Border Router as a router that "attaches to more than one area."

2. Juniper Networks Documentation. (2023). OSPF Areas Overview. "Area border routers (ABRs) are routers that connect the OSPF backbone to the nonbackbone areas."