Implementing a Scalable Multiarea Network OSPF-Based Solution

© 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-1

Implementing a Scalable Multiarea Network OSPF-Based Solution

Configuring and Verifying OSPF Special Area Types


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


Types of Areas


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


Stub and Totally Stub Area RulesAn 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


OSPF Stub Areas External LSAs are stopped. The default route is advertised into the stub area by the ABR.


Stub Area Configuration

This command turns on stub area networking. Configure all routers in the stub area as stub routers.

This command defines the cost of a default route sent into the stub area (default is 1); defining the cost is optional.

area 2 stub

R2(config-router)#

area 2 stubarea 2 default-cost 10

ABR(config-router)#


OSPF Stub Area Configuration Example


OSPF Totally Stubby Areas External and Summary LSAs are stopped The default route is sent instead Cisco proprietary feature


Totally Stubby Area Configuration

This command turns on stub area networking Configure all routers in the stub area as stub routers

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

area 2 stubR2(config-router)#

area 2 stub no-summaryarea 2 default-cost 10

ABR(config-router)#


Router R2 is the preferred ABR

Totally Stubby Configuration Example


Routing Table in a Normal Area


Routing Table in a Stub Area Use the area 1 stub command to configure a stub area.


Routing Table in a Stub Area with Summarization Use the area 1 stub and area 1 range commands.


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.


OSPF Not-So-Stubby Areas (NSSAs) NSSA breaks stub area rules ASBR is allowed inside LSA type 7 sent by ASBR

ABR converts LSA type 7 to LSA type 5

ABR sends the default route into NSSA instead of external (LSA type 5) routes


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


Totally NSSA Area Configuration

This command turns on NSSA area networking Set on all routers in the NSSA area

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)

area 2 nssa

R2(config-router)#

area 2 nssa no-summaryarea 2 default-cost 10

ABR(config-router)#


NSSA Configuration Example


Totally NSSA Configuration Example


Example of Different Areas


show ip ospf

show Commands for Stub and NSSA

Displays which areas are normal, stub, or NSSA

Displays the details of the LSAs show ip ospf database

show ip ospf database nssa-external

show ip route

R1#

R1#

R1#

R1#

Displays the details of each LSA type 7 update in the database

Displays all routes


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.


Implementing a Scalable Multiarea Network OSPF-Based Solution

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