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. Figure 2-9 shows a typical OSPF network, with multiple areas containing the different types of OSPF routers.

Figure 2-9 Types of OSPF Routers

Link to Another ' Autonomous i System (AS) !

*

f i

IR

ABRl r Area

ASBR

ABRl

AreaO OSPF Backbone

Area 3

IR = Internal Router

ABR = Area Border Router

BR = Backbone Router

ASBR = Autonomous System Border Router

IR = Internal Router

ABR = Area Border Router

BR = Backbone Router

ASBR = Autonomous System Border Router

78 Chapter 2: Introduction to OSPF

The following sections provide general descriptions for the four types of OSPF routers.

Internal 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. For additional information on this protocol and its use, see Internet Routing Architectures, Second Edition, by Sam Halabi.

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.

Backbone Routers

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.

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