OSPF Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Release

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OSPF Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Release

OSPF areas

Updated: June 26, 2026

Explains how OSPF areas segment an autonomous system to enhance scalability, improve routing efficiency, and control traffic flow, while also describing router roles that manage internal and external connectivity.

An OSPF area is a network architecture concept that

  • divides an OSPF autonomous system into multiple areas to improve scalability and reduce routing overhead,

  • assigns routers specific roles—such as interior routers, area border routers (ABRs), and autonomous system boundary routers (ASBRs), and

  • defines how routing information is distributed within and across areas to control traffic flow and manage external connectivity.

OSPF area types

  • Backbone area (Area 0): Distributes routing information between all areas and uses the reserved area ID 0.0.0.0.

  • Stub area: Does not accept external route advertisements and typically has only one router connecting it to the rest of the autonomous system.

  • Not-so-Stubby Area (NSSA): Similar to a stub area, but allows limited import of external routes (Type 7 LSAs) and supports redistribution of routes from other protocols.

Router roles

  • Interior router: Attaches to a single area; all interfaces reside within that area.

  • Area border router (ABR): Connects multiple areas, runs separate OSPF processes for each, and summarizes area information to the backbone area.

  • Autonomous system boundary router (ASBR): Connects OSPF with other autonomous systems and redistributes routes from external protocols (such as BGP) into OSPF using Type 5 and Type 7 LSAs.

Additional reference information

  • Areas subdivide an OSPF autonomous system into smaller, manageable segments, which help reduce routing traffic and the size of the link-state database.

  • OSPF routers within an area only see their area’s topology, limiting unnecessary flooding of link-state information.

  • ABRs send configuration summaries for their attached areas to the backbone, supporting efficient interarea routing.

  • ASBRs connect the OSPF network to other autonomous systems, ensuring the exchange of external routing information.

  • Stub ABRs advertise a single default route to stub areas, conserving database space.

  • NSSA ABRs translate Type 7 LSAs into Type 5 LSAs for propagation through the routing domain.

  • Autonomous systems are collections of networks under unified administrative control and may include multiple OSPF areas.