Asbr stands for Autonomous System Boundary Router. In the Open Shortest Path First (OSPF) protocol, an ASBR is a specialized router that acts as a gateway, connecting an OSPF network to other routing domains, or "autonomous systems" (AS). An ASBR's primary function is to inject external routes into the OSPF domain through a process called route redistribution.
The role of an ASBR
An ASBR is a crucial component in networks that must communicate beyond their own OSPF routing domain. Its functions and characteristics include:
- Redistribution: The defining characteristic of an ASBR is its ability to redistribute routes learned from another routing protocol—such as BGP, EIGRP, RIP, or even static routes—into the OSPF network. This is how networks outside of the OSPF autonomous system become reachable.
- Location: Unlike an Area Border Router (ABR) which must connect to the OSPF backbone (Area 0), an ASBR can be located in any type of OSPF area, including the backbone, standard areas, or Not-So-Stubby Areas (NSSAs).
- External LSA Generation: To advertise the external routes it has learned, an ASBR generates specific types of Link-State Advertisements (LSAs):
- Type 5 External LSA: In a standard OSPF area, the ASBR floods a Type 5 LSA to advertise the external routes.
- Type 7 NSSA External LSA: In a Not-So-Stubby Area (NSSA), the ASBR creates a special Type 7 LSA for external routes. This LSA is then converted into a Type 5 LSA by the NSSA's ABR before being flooded to the rest of the OSPF domain.
Comparison with an Area Border Router (ABR)
It is important to distinguish an ASBR from an ABR, which also functions at a network boundary. The two have fundamentally different roles:
| Feature | ASBR (Autonomous System Boundary Router) | ABR (Area Border Router) |
|---|---|---|
| Function | Connects an OSPF domain to an external routing domain (e.g., another OSPF domain, EIGRP, BGP). | Connects an OSPF area to the OSPF backbone (Area 0), managing traffic between different areas within the same OSPF domain. |
| Routing Information | Imports and exports routing information between OSPF and non-OSPF networks. | Summarizes and distributes routing information between OSPF areas. |
| LSA Type | Generates Type 5 (External) LSAs in normal areas or Type 7 (NSSA External) LSAs in NSSAs. | Generates Type 3 (Summary) LSAs to represent networks in other areas. |
| Location | Can be in any OSPF area. | Sits at the boundary of an area, with at least one interface in Area 0. |
| Role in OSPF | Facilitates communication with external routing protocols. | Maintains OSPF's hierarchical structure and reduces routing table size. |
Example use case
Consider a corporate network running OSPF for its internal routing. The company connects to a partner's network which uses EIGRP, as well as to the public internet via a BGP connection. In this scenario:
- A router that connects the OSPF network to the partner's EIGRP network is configured as an ASBR. It redistributes the EIGRP routes into the OSPF domain so that OSPF routers can send traffic to the partner.
- The border router connecting the OSPF network to the public internet via BGP is also an ASBR. It redistributes BGP-learned routes from the internet into the OSPF domain.
OSPF area types and ASBRs
The type of OSPF area where the ASBR resides affects how external routes are handled.
- Standard areas: External routes are injected as Type 5 LSAs and flooded throughout the entire OSPF domain.
- Stub areas: This area type is designed to minimize LSA traffic by blocking all external LSAs (Type 5s). As a result, ASBRs are not allowed within a stub area.
- Totally stubby areas: Even more restrictive than a stub area, totally stubby areas block both external (Type 5) and inter-area (Type 3) LSAs, replacing them with a single default route. No ASBRs can exist here.
- Not-So-Stubby Areas (NSSA): NSSAs are a special type of stub area that allows ASBRs to exist within them. The ASBR generates a Type 7 LSA for external routes, which is then translated by the NSSA's ABR into a Type 5 LSA to be flooded into the backbone.
A router as both ABR and ASBR
It is possible for a single router to serve as both an ABR and an ASBR. This occurs when a router connects two different OSPF areas (making it an ABR) while also redistributing routes from an external routing protocol (making it an ASBR).