area [id] nssa
Configures an OSPF area as a Not-So-Stubby Area (NSSA), allowing external routes from outside the OSPF domain to be imported as Type 7 LSAs while still blocking most external routes from other areas.
area [id] nssaWhen to Use This Command
- Connect a remote site with a single router to an ISP using OSPF, where the remote site needs to redistribute a default route or a few external routes without receiving all external routes from the backbone.
- Integrate a non-OSPF network (e.g., EIGRP) into an OSPF stub area by redistributing external routes into the NSSA, allowing those routes to be advertised as Type 7 LSAs.
- Create a stub area that still needs to import a limited set of external routes (e.g., a default route) from an external source, such as a router connected to the internet.
- Connect two OSPF domains where one domain uses NSSA to inject external routes into the other domain without flooding Type 5 LSAs.
Command Examples
Configure area 1 as NSSA
area 1 nssaR1(config-router)# area 1 nssa R1(config-router)#
No output is displayed upon successful configuration. The command simply enables NSSA for area 1.
Configure area 2 as NSSA with no redistribution
area 2 nssa no-redistributionR1(config-router)# area 2 nssa no-redistribution R1(config-router)#
The 'no-redistribution' keyword prevents the NSSA ABR from redistributing Type 7 LSAs into Type 5 LSAs. This is useful when you want to keep external routes within the NSSA only.
Understanding the Output
The 'area [id] nssa' command itself does not produce any output. To verify the NSSA configuration, use 'show ip ospf' or 'show ip ospf interface'. In 'show ip ospf', look for the line 'Area is NSSA' under the specific area. In 'show ip ospf interface', the output will include 'Area is NSSA' if the interface belongs to an NSSA. Additionally, 'show ip ospf database nssa-external' displays Type 7 LSAs. A healthy NSSA will have Type 7 LSAs for external routes, and the ABR will translate them to Type 5 LSAs (unless 'no-redistribution' is configured). Watch for missing Type 7 LSAs if redistribution is not working, or for Type 5 LSAs leaking into the NSSA if the ABR is misconfigured.
CCNA Exam Tips
Remember that NSSA allows Type 7 LSAs for external routes, which are translated to Type 5 by the ABR. This is a key distinction from stub areas that block all external routes.
The 'no-redistribution' keyword prevents the ABR from translating Type 7 to Type 5. This is a common exam scenario to test your understanding of NSSA behavior.
NSSA is the only area type that can import external routes while still being a stub-like area. Expect questions comparing NSSA to stub and totally stubby areas.
On the CCNA exam, you may be asked to configure NSSA to allow redistribution of a default route from an external source into an OSPF area.
Common Mistakes
Mistake: Configuring NSSA on an area that already has virtual links. NSSA does not support virtual links.
Mistake: Forgetting that NSSA ABRs translate Type 7 to Type 5 by default, which may cause external routes to leak into other areas unintentionally.
Mistake: Using 'area [id] nssa' on an area that is not a stub area; NSSA is a variant of stub area and inherits stub area restrictions (e.g., no Type 5 LSAs).
Related Commands
show ip ospf
Displays general information about OSPF routing process, including router ID, areas, and LSDB statistics, used to verify OSPF configuration and operational status.
show ip ospf database
Displays the OSPF link-state database (LSDB) to verify OSPF adjacencies, check for missing or corrupt LSAs, and troubleshoot OSPF routing issues.
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