OSPF - Autonomous System Boundary Router (ASBR)

In OSPF an Autonomous System Boundary Router (ASBR) is a router that connects an OSPF autonomous system (AS) to other routing domains, such as another OSPF area, a different routing protocol (e.g., EIGRP, BGP), or an external network. The role of an ASBR is to import or redistribute external routes into the OSPF domain.

Key Points About ASBRs

1. Route Redistribution: ASBRs are used to inject routes learned from outside the OSPF domain into the OSPF network. This process is called route redistribution.

2. External Routes: These redistributed routes are known as OSPF External Routes and are classified as either Type 1 (E1) or Type 2 (E2):

   • E1 routes consider both the external metric (from the other protocol) and the internal OSPF path cost.
   • E2 routes use only the external metric, making the internal OSPF path cost irrelevant.

3. LSA Type 5 (External LSA): An ASBR generates Type 5 LSAs to advertise these external routes throughout the OSPF domain, except for stub areas.

4. Location in the Network: ASBRs can be located in any OSPF area (e.g., backbone area, standard area, or even NSSA areas), but the types of LSAs and area types dictate how external routes are handled and propagated.

Example Use Case:

If an OSPF network needs to learn routes from a BGP network or a static route to an external subnet, the ASBR will import these routes into the OSPF network, allowing OSPF routers to reach destinations outside their autonomous system.

Links

https://networklessons.com/ospf/introduction-to-ospf