1. © 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

2. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-2 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

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

4. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-4 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

5. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-5 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

6. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-6 OSPF Stub Areas  External LSAs are stopped.  The default route is advertised into the stub area by the ABR.

7. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-7 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 stub area 2 default-cost 10 ABR(config-router)#

8. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-8 OSPF Stub Area Configuration Example

9. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-9 OSPF Totally Stubby Areas  External and Summary LSAs are stopped  The default route is sent instead  Cisco proprietary feature

10. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-10 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 stub R2(config-router)# area 2 stub no-summary area 2 default-cost 10 ABR(config-router)#

11. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-11  Router R2 is the preferred ABR Totally Stubby Configuration Example

12. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-12 Routing Table in a Normal Area

13. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-13 Routing Table in a Stub Area  Use the area 1 stub command to configure a stub area.

14. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-14 Routing Table in a Stub Area with Summarization  Use the area 1 stub and area 1 range commands.

15. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-15 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.

16. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-16 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

17. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-17 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

18. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-18 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-summary area 2 default-cost 10 ABR(config-router)#

19. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-19 NSSA Configuration Example

20. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-20 Totally NSSA Configuration Example

21. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-21 Example of Different Areas

22. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-22 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#  Displays the details of each LSA type 7 update in the database  Displays all routes

23. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-23 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.

24. © 2009 Cisco Systems, Inc. All rights reserved. ROUTE v1.0—3-24