Configuring and
Verifying OSPF
Special Area Types
Implementing a Scalable Multiarea Network OSPFBased Solution
© 2009 Cisco Systems, Inc. All rights reserved.
ROUTE v1.0—3-1
OSPF Area Types and Structure
 OSPF is based on a two-level hierarchical area structure
 Each area has its own topology database
 Area Types
– Backbone area: Connects all other areas
– Normal area: Contains all of the internal and external routing
information
– Stub area: Contains internal and area routing information, but
not external routing information
– Totally stubby area: Contains area routing information only;
Cisco proprietary
– Not-so-stubby area: Contains area and external routing
information
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Types of Areas
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OSPF Router and LSA Types
 ABR is generating Summary LSAs
 ASBR is generating External LSAs
 Summary and External LSAs can be blocked and default route is
sent instead
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Stub and Totally Stub Area Rules
An area can be stub or totally stub if:
 There is one ABR or more
 All routers that are members of the stub area are configured as
stub routers
 There is no ASBR in the area
 The area is not an area 0
 No virtual links go through the area
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OSPF Stub Areas
 External LSAs are stopped.
 The default route is advertised into the stub area by the ABR.
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Stub Area Configuration
R2(config-router)#
area 2 stub
 This command turns on stub area networking.
 Configure all routers in the stub area as stub routers.
ABR(config-router)#
area 2 stub
area 2 default-cost 10
 This command defines the cost of a default route sent into the
stub area (default is 1); defining the cost is optional.
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OSPF Stub Area Configuration Example
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OSPF Totally Stubby Areas
 External and Summary LSAs are stopped
 The default route is sent instead
 Cisco proprietary feature
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Totally Stubby Area Configuration
R2(config-router)#
area 2 stub
 This command turns on stub area networking
 Configure all routers in the stub area as stub routers
ABR(config-router)#
area 2 stub no-summary
area 2 default-cost 10
 First command defines the totally stubby area on the ABR router
 Second command defines the cost of a default route sent into the
totally stubby area (default is 1); defining the cost is optional
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Totally Stubby Configuration Example
 Router R2 is the preferred ABR
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Routing Table in a Normal Area
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Routing Table in a Stub Area
 Use the area 1 stub command to configure a stub area.
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Routing Table in a Stub Area with
Summarization
 Use the area 1 stub and area 1 range commands.
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Routing Table in a Totally Stubby Area
 Use the area 1 stub command on all internal routers.
 Use the area 1 stub no-summary command on the ABRs.
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OSPF Not-So-Stubby Areas (NSSAs)
 NSSA breaks stub area rules
 ASBR is allowed inside
 LSA type 7 sent by ASBR
© 2009 Cisco Systems, Inc. All rights reserved.
 ABR converts LSA type 7
to LSA type 5
 ABR sends the default route
into NSSA instead of external
(LSA type 5) routes
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OSPF Totally NSSA Areas
 ABR is blocking Type 3, 4, 5 LSAs
 ABR is sending the default route into the NSSA instead
 This is a Cisco proprietary feature
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Totally NSSA Area Configuration
R2(config-router)#
area 2 nssa
 This command turns on NSSA area networking
 Set on all routers in the NSSA area
ABR(config-router)#
area 2 nssa no-summary
area 2 default-cost 10
 The first command defines the totally NSSA area on ABRs
 The second command defines the cost of a default route sent into
the NSSA area (default is 1)
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NSSA Configuration Example
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Totally NSSA Configuration Example
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Example of Different Areas
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show Commands for Stub and NSSA
R1#
show ip ospf
 Displays which areas are normal, stub, or NSSA
R1#
show ip ospf database
 Displays the details of the LSAs
R1#
show ip ospf database nssa-external
 Displays the details of each LSA type 7 update in the database
R1#
show ip route
 Displays all routes
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Summary
 There are several OSPF area types: normal, backbone, stub,
totally stubby, NSSA, and totally NSSA.
 Use the area area-id stub command to define an area as stubby.
 Use the area area-id stub command with the no-summary
keyword on the ABR only to define an area as totally stubby.
 For stub areas, external routes are not visible in the routing table,
but are accessible via the intra-area default route. For totally
stubby areas, interarea and external routes are not visible in the
routing table, but are accessible via the intra-area default route.
 Use the area area-id nssa command to define an area as NSSA.
 Use the show ip ospf, show ip ospf database, and show ip
route commands to verify all types of stub areas. Use the show
ip ospf database nssa-external command to display details of
type 7 LSAs.
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