Chapter 36 OSPF

The following table describes the four classes of OSPF routers.

Table 113 OSPF: Router Types

TYPE

DESCRIPTION

Internal Router (IR)

An Internal or intra-area router is a router in an area.

 

 

Area Border Router

An Area Border Router connects two or more areas.

(ABR)

 

 

 

Backbone Router (BR)

A backbone router has an interface to the backbone.

 

 

AS Boundary Router

An AS boundary router exchanges routing information with

 

routers in other ASs.

 

 

The following figure depicts an OSPF network example. The backbone is area 0 with a backbone router. The internal routers are in area 1 and 2. The area border routers connect area 1 and 2 to the backbone.

Figure 162 OSPF Network Example

36.1.2 How OSPF Works

Layer-3 devices exchange routing information to build a synchronized link state database within the same AS or area. The link state database contains records of router IDs, their associated links and path costs. Each device can then use the link state database and Dijkstra algorithm to compute the least cost paths to network destinations.

Layer-3 devices build a synchronized link state database by exchanging Hello messages to confirm which neighbor (layer-3) devices exist and then they exchange database descriptions (DDs) to create the link state database. The link state database is constantly updated through LSAs (Link State Advertisements).

36.1.3 Interfaces and Virtual Links

An OSPF interface is a link between a layer-3 device and an OSPF network. An interface has state information, an IP address and subnet mask associated with it.

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