Lenovo Flex System Fabric CN4093, Application Manual

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CN4093 Application Guide for N/OS 8.4
Types of OSPF Areas
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 47 , 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.
Figure 47.
OSPF Area Types
Backbone
Area 0
Stub Area
Not-So-Stubby Area
(NSSA)
Transit Area
No External Routes
from Backbone
Stub Area, NSSA,
or Transit Area
Connected to Backbone
via Virtual Link
(Also a Transit Area)
External LSA
Routes
Internal LSA
Routes
ABR ABR
ABR
ASBR
Non-OSPF Area
RIP/BGP AS
Virtual
Link
ABR
ABR = Area Border Router
ASBR = Autonomous System
Boundary Router