ASBR

An ASBR, or Autonomous System Boundary Router, is a router that connects an OSPF (Open Shortest Path First) routing domain to another routing domain or autonomous system and injects external routes into OSPF. It serves as the point where routes learned outside the OSPF domain enter the OSPF routing process.

In OSPF, external routes originate from other routing protocols (such as BGP, RIP, or EIGRP) or from static routes that are redistributed into OSPF by the ASBR. In standard OSPF areas, the ASBR advertises these external destinations as type 5 external LSAs. In Not-So-Stubby Areas (NSSAs), an ASBR can inject type 7 LSAs, which are translated to type 5 LSAs by the Area Border Router when exiting the NSSA.

ASBRs differ from ABRs. An ABR is an Area Border Router that connects multiple OSPF areas and distributes inter-area routes within the OSPF domain. An ASBR, by contrast, connects the OSPF domain to external routing domains and injects external routes into OSPF, regardless of the number of internal areas.

Operational considerations include route redistribution control, filtering, and metrics. Redistribution can introduce suboptimal paths or routing loops if not carefully managed. Administrators often use route maps, distribute-lists, or other policy controls to select which external routes are advertised and to adjust external route metrics and administrative distance. Redundancy with multiple ASBRs is common to improve resilience.