What is the definition of an OSPF area border router?

An area border router (ABR) in OSPF (Open Shortest Path First) is defined as a router that has interfaces in multiple OSPF areas. This key characteristic allows the ABR to play a vital role in the OSPF network by facilitating the routing of traffic between different areas.

The purpose of an ABR is to connect one or more OSPF areas to the backbone area (Area 0) and to help manage the distribution of routing information within each area. The ABR summarizes the routing information it learns from non-backbone areas and sends it to the backbone area. This summarization reduces the amount of routing information exchanged, simplifying the routing topology and contributing to overall network efficiency.

In summary, option A accurately describes an area border router by highlighting its essential function of connecting multiple OSPF areas, which is fundamental to the efficient operation and structure of OSPF routing protocols.