ASBRs

An autonomous system boundary router (ASBR) is a router that connects an OSPF routing domain to external routing domains and redistributes routes between them. The ASBR is typically located at the edge of an OSPF Autonomous System (AS) and is responsible for injecting routes learned from other protocols into OSPF, and potentially advertising OSPF routes into those other protocols.

Within OSPF, an ASBR originates External Route LSAs (Type-5 in standard areas and Type-7 in NSSA areas) to advertise routes from external sources. It can redistribute routes from other routing protocols (such as BGP, RIP, or static routes) into OSPF or import OSPF routes into the external protocol.

Redistribution is configurable: the administrator selects which routes to redistribute, assigns external metrics (E1 or E2), and may apply route maps or filters to control scope. E1 external routes incur cost from the ASBR to the destination plus internal cost, whereas E2 routes carry the external cost with internal costs only affecting path selection. NSSA areas use Type-7 LSAs that can be translated to Type-5 by an ABR, enabling redistribution across the boundary.

ASBRs require careful planning to avoid routing loops, instability, or suboptimal paths. Filtering and proper route tagging help prevent unwanted redistribution. In large networks, multiple ASBRs may exist for redundancy or for distributing different external route sets.

Examples of ASBR deployment include a border router that redistributes BGP routes learned from an Internet edge into an internal OSPF domain, or a router that redistributes static routes into OSPF to reach networks learned via another protocol.