What is the role of an ABR in OSPF?

• A. An ABR designates a default route on the router.
• B. An ABR summarizes routes between OSPF areas and connects multiple areas to the backbone.
• C. An ABR routes traffic between different autonomous systems.
• D. An ABR changes the router ID during stabilization.

Answer: (B)

An ABR serves as the border between OSPF areas and the backbone. It connects multiple OSPF areas to Area 0, keeping separate routing information for each area and handling the exchange of routes between them. The key job is inter-area route summarization: an ABR takes routes learned in one area and advertises them into other areas as summarized inter-area routes (Type-3 LSAs), allowing devices in different areas to reach destinations without knowing every detail of every area. This backbone-centric pathing is what enables scalable routing in OSPF.

The other options don’t fit this role. ABRs don’t designate default routes (that's handled by other mechanisms). They’re not responsible for routing between different autonomous systems (that’s BGP’s job). They also don’t change the router ID during stabilization.