Ospfv2 Overview; Types Of Ospf Areas - Lenovo RackSwitch G8264 Application Manual

OSPFv2 Overview

Types of OSPF Areas

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G8264 Application Guide for ENOS 8.4
OSPF is designed for routing traffic within a single IP domain called an
Autonomous System (AS). The AS can be divided into smaller logical units known
as areas.
All routing devices maintain link information in their own Link State Database
(LSDB). OSPF allows networks to be grouped together into an area. The topology
of an area is hidden from the rest of the AS, thereby reducing routing traffic.
Routing within an area is determined only by the area's own topology, thus
protecting it from bad routing data. An area can be generalized as an IP
subnetwork.
The following sections describe key OSPF concepts.
An AS can be broken into logical units known as areas. In any AS with multiple
areas, one area must be designated as area 0, known as the backbone. The backbone
acts as the central OSPF area. All other areas in the AS must be connected to the
backbone. Areas inject summary routing information into the backbone, which
then distributes it to other areas as needed.
As shown in Figure 48, OSPF defines the following types of areas:
Stub Area—an area that is connected to only one other area. External route

information is not distributed into stub areas.
 Not‐So‐Stubby‐Area (NSSA)—similar to a stub area with additional capabilities.
Routes originating from within the NSSA can be propagated to adjacent transit
and backbone areas. External routes from outside the AS can be advertised
within the NSSA but can be configured to not be distributed into other areas.
 Transit Area—an area that carries data traffic which neither originates nor
terminates in the area itself.