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Implementing a Scalable Multiarea Network OSPF-Based Solution

Configuring and Verifying OSPF Special Area Types. Implementing a Scalable Multiarea Network OSPF-Based Solution. OSPF Area Types and Structure. OSPF is based on a two-level hierarchical area structure Each area has its own topology database Area Types

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Implementing a Scalable Multiarea Network OSPF-Based Solution

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  1. Configuring and Verifying OSPF Special Area Types Implementing a Scalable Multiarea Network OSPF-Based Solution

  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. Types of Areas

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

  7. Stub Area Configuration • This command turns on stub area networking. • Configure all routers in the stub area as stub routers. R2(config-router)# area 2 stub 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.

  8. OSPF Stub Area Configuration Example

  9. OSPF Totally Stubby Areas • External and Summary LSAs are stopped • The default route is sent instead • Ciscoproprietary feature

  10. Totally Stubby Area Configuration • This command turns on stub area networking • Configure all routers in the stub area as stub routers R2(config-router)# area 2 stub 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

  11. Totally Stubby Configuration Example • Router R2 is the preferred ABR

  12. Routing Table in a Normal Area

  13. Routing Table in a Stub Area • Use thearea 1 stub command to configure a stub area.

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

  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. 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. 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. Totally NSSA Area Configuration R2(config-router)# • This command turns on NSSA area networking • Set on all routers in the NSSA area area 2 nssa 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)

  19. NSSA Configuration Example

  20. Totally NSSA Configuration Example

  21. Example of Different Areas

  22. show Commands for Stub and NSSA R1# • Displays which areas are normal, stub, or NSSA show ip ospf R1# show ip ospf database R1# • Displays the details of the LSAs 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

  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.

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