Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 120
Router Types

When there is only one area, there is basically only one specialized type of router: the ones that deal with external routes. When an OSPF environment is split into multiple areas, multiple routers are required. There are six types of routers in an OSPF environment:

Backbone Router (BR): A router that has an interface to the backbone.

Area Border Router (ABR): A router that has interfaces to multiple areas.

Autonomous System Boundary Router (ASBR): A router that exchanges routing information with routers that are attached to different autonomous systems.

Internal Router (IR): A router whose attachments all belong to the same area.

Designated Router (DR): One router on a subnet that is selected as the designated router. All other routers on the subnet form an adjacency (a logical point-to-point connection on a subnet) to this router. Information about networks to and from the subnet is transferred over the DR. The DR generates network LSA on behalf of its subnet and floods this information throughout its area. This advertisement in the DR identifies all routers adjacent to this DR and records the link-states of all the routers currently attached to the network.

Router Types

Backup Designator Router (BDR): Backs up the DR in case the DR fails.

Some of these router types have overlapping roles. For example, an ABR can also be a backbone router.