What is the primary purpose of an ABR in OSPF?

The primary purpose of an Area Border Router (ABR) in OSPF (Open Shortest Path First) is indeed to summarize routes between areas. ABRs connect different OSPF areas to the backbone area (Area 0), and they play a vital role in managing the distribution of routing information between these areas.

When multiple areas are connected in an OSPF network, the ABR can aggregate or summarize the routing information from one area before passing it into another. This route summarization reduces the amount of routing information that needs to be shared across the network, leading to more efficient use of bandwidth and a more streamlined routing table. By summarizing routes, the ABR helps to contain the OSPF database size and decreases the resource burden on routers within the areas.

In broader terms, while the other choices provide relevant functions associated with OSPF, they do not encompass the specific role of the ABR in summarizing routes. For example, while an ABR does connect external networks to the OSPF domain, this is not its primary function. Its principal role is to facilitate efficient communication among areas through summarization, which is crucial for maintaining optimal routing practices within large and complex OSPF networks.