Types of OSPF Routers
Four different types of routers designate the hierarchical routing structure used by OSPF. Each router has a unique role and set of defining characteristics within the hierarchy.
The following sections provide general descriptions for the four types of OSPF routers.
Internal routers (IRs) are routers whose directly connected networks all belong to the same OSPF area. These types of routers have a single link-state database because they belong to only one area.
Area Border Routers
ABRs are attached to multiple OSPF areas, so there can be multiple ABRs within a network. ABRs have multiple instances of the link-state database because of this. The ABR has one database for each area that is summarized and then presented to the backbone for distribution to other areas.
Routers located on the border of one or more OSPF areas and that connect those areas to the backbone network are known as ABRs, which are considered to be members of both the OSPF backbone and the attached areas. The ABRs therefore maintain multiple link-state databases that describe both the backbone topology and the topology of the other areas. An ABR sends summary LSAs to the backbone area, and to be considered an ABR, the router must be connected to the backbone.
Autonomous System Boundary Routers
ASBRs are connected to more than one AS and exchange routing information with routers in another AS. ASBRs advertise the exchanged external routing information throughout their AS. Every router within an AS knows how to get to each ASBR with its AS. ASBRs run both OSPF and another routing protocol, such as RIP or BGP. ASBRs must reside in a nonstub OSPF area.
TIP When using Cisco routers, the redistribution command is often used to bring two routing protocols together. BGP can also be used to bring multiple autonomous systems together.
ASBRs deal with external routes. One way that an ASBR can be configured or activated in OSPF is to add the redistribute static or redistribute connected command within the OSPF routing process.
Routers whose interfaces connect them only to the backbone area are considered backbone routers (BRs). BRs do not have an interface to the other OSPF areas, because if they did, they would be considered ABRs.
In this tutorial:
- OSPF Basics
- What Is a Routing Protocol?
- Basic Routing Protocol Operation
- Link-State Versus Distance Vector Routing Protocols
- Link-State Routing Protocols
- OSPF Characteristics
- Integrated Intermediate System-to-Intermediate System
- Distance Vector Routing Protocols
- Selecting a Routing Protocol
- Operational Considerations
- Protocols Supported
- Routing Hierarchies
- IP Address Management
- IP Encapsulation Support
- Technical Considerations
- Routing Updates
- Business Considerations
- SPF Overview
- OSPF Routing Hierarchy
- Hierarchical Network Design Techniques
- Routing Types Within an OSPF Network
- External Routes
- OSPF Areas
- Characteristics of a Standard OSPF Area
- Stub Areas
- Not-So-Stubby Areas
- OSPF Operational Environment
- Types of OSPF Routers
- OSPF Network Types
- Router Identification