Must every router in a stub or NSSA area use the same area type?

Yes. The routers must agree on the stub or NSSA option. A mismatch prevents the OSPF adjacency from reaching Full.

OSPF Stub Areas - Networking Essentials

Q: What is the main purpose of an OSPF stub area?

A stub area reduces external route detail. The ABR blocks Type 4 and Type 5 LSAs and gives routers a default route toward the rest of the OSPF domain.

Q: What is the difference between stub and totally stubby areas?

A stub area still receives inter-area Type 3 summaries. A totally stubby area suppresses most Type 3 summaries too and keeps only a Type 3 default route from the ABR.

Q: When should I use an NSSA?

Use an NSSA when an edge area should block external Type 5 LSAs from the rest of the domain but a router inside that area must redistribute a local external route as a Type 7 LSA.

Q: Does an NSSA receive a default route automatically?

A regular NSSA does not automatically receive a default route on Cisco IOS. Configure default-information-originate on the ABR when the design needs one. A totally NSSA receives a Type 3 default route with no-summary behavior.

Stub Areas at a Glance

A stub design keeps unnecessary external route information out of an edge area. Routers use a default route toward the Area Border Router (ABR) instead of storing every external path.

Stub blocksType 4 and 5

Totally stubby also blocksMost Type 3

NSSA local externalType 7

Exit pathDefault via ABR

Choose by requirement: Do not select the area with the smallest table automatically. First decide whether the area needs inter-area detail or local redistribution.

1. Read the Multi-Area Lab

R2 is the ABR between Area 0 and edge Area 10. R1 advertises an external route from the backbone side. R3 is the edge router whose routing table we want to simplify.

Area 0 · Backbone

External network203.0.113.0/24

R1 · ASBRCreates Type 5 LSA

R2 · ABRControls what enters Area 10

Area 10 · Edge Area

R3Uses ABR as the exit

Branch LAN10.10.30.0/24

The lab changes only Area 10. Compare R3's LSDB and route table before and after each area-type change.

Baseline first: Begin with Area 10 as a normal area. Capture the Type 3, Type 4, and Type 5 LSAs and the routes R3 learns before applying stub behavior.

2. Connect Area Types to LSAs

An area type changes which LSAs may cross the ABR. Learn the role of each LSA before memorizing commands.

LSA	Purpose in this lesson	Key boundary behavior
Type 1	Describes a router and its links inside one area	Never crosses an ABR
Type 2	Describes a broadcast segment and attached routers	Never crosses an ABR
Type 3	Advertises networks from another OSPF area	Allowed in stub/NSSA; mostly suppressed by no-summary designs
Type 4	Shows how to reach an ASBR in another area	Blocked from stub and NSSA areas
Type 5	Carries external routes across normal OSPF areas	Blocked from stub and NSSA areas
Type 7	Carries a locally redistributed route inside an NSSA	Translated to Type 5 when leaving the NSSA

Type 1 and Type 2 LSAs still describe the local area. Stub behavior mainly changes external information and, with no-summary, inter-area summaries.

3. Compare Every Area Type

Area type	Inter-area Type 3	Type 4/5 from outside	Local ASBR	Default route
Standard	Allowed	Allowed	Allowed with Type 5	Only when configured
Stub	Allowed	Blocked	Not allowed	Automatic Type 3 from ABR
Totally stubby	Most suppressed	Blocked	Not allowed	Automatic Type 3 from ABR
NSSA	Allowed	Blocked	Allowed with Type 7	Configure when required
Totally NSSA	Most suppressed	Blocked	Allowed with Type 7	Type 3 from ABR

Vendor note: Totally stubby and totally NSSA are common Cisco terms implemented with no-summary. Confirm equivalent support and syntax on other platforms.

4. Choose the Correct Design

Question 1

Must a router inside the area redistribute routes?

Yes: choose NSSA or totally NSSA. No: stub remains an option.

Question 2

Does the area need specific inter-area routes?

Yes: use stub or NSSA. No: consider a no-summary design.

Question 3

Is a default route sufficient for every exit?

Confirm forwarding and return paths. A smaller table is useful only when the default path is correct.

Area 0 cannot be stub. The backbone must carry information between other areas.
A virtual link cannot cross a stub area. Choose the transit-area design before enabling stub behavior.
Multi-exit areas need care. A default route can hide path detail and create an unexpected exit.

5. Configure Stub and Totally Stubby Areas

Every OSPF router inside Area 10 must agree that it is a stub. Only the ABR receives no-summary for a totally stubby design.

R2 ABR · Stub

router ospf 1
 area 10 stub

R3 internal router · Stub

router ospf 1
 area 10 stub

R2 ABR · Totally stubby

router ospf 1
 area 10 stub no-summary

R3 stays stub

router ospf 1
 area 10 stub

What changes on R3? Stub removes external O E1/O E2 routes and supplies O*IA 0.0.0.0/0. Totally stubby also removes most specific O IA routes.

6. Configure NSSA and Totally NSSA

Use NSSA when R3 must redistribute a branch-only route while Area 10 still blocks Type 5 LSAs arriving from the rest of the domain.

R2 ABR · NSSA with default

router ospf 1
 area 10 nssa default-information-originate

R3 NSSA ASBR

ip route 172.20.50.0 255.255.255.0 Null0
!
router ospf 1
 area 10 nssa
 redistribute static subnets

R2 ABR · Totally NSSA

router ospf 1
 area 10 nssa no-summary

R3 remains NSSA

router ospf 1
 area 10 nssa
 redistribute static subnets

Do not mix flags: All routers must agree on stub versus NSSA. A normal NSSA needs default-information-originate on the ABR when the area requires a default route.

7. Follow Type 7 to Type 5 Translation

R3 creates the redistributed prefix as a Type 7 LSA because Type 5 LSAs are not permitted inside an NSSA. An NSSA translator—normally an ABR—changes it to Type 5 for the rest of the OSPF domain.

1

R3 redistributes

The NSSA ASBR creates a Type 7 LSA for 172.20.50.0/24.

2

R2 translates

The elected translator converts the Type 7 LSA into a Type 5 LSA.

3

Area 0 receives it

Routers outside the NSSA learn the external prefix from the Type 5 LSA.

Inside Area 10

R3# show ip ospf database nssa-external

Type-7 AS External Link States
Link State ID: 172.20.50.0
Advertising Router: 3.3.3.3

Outside the NSSA

R1# show ip ospf database external

Type-5 AS External Link States
Link State ID: 172.20.50.0
Advertising Router: 2.2.2.2

8. Verify and Troubleshoot the Design

Prove adjacency first, LSA filtering second, and usable forwarding last.

Neighbors: show ip ospf neighbor must show Full. An adjacency failure after the change usually means area flags do not match.
Area options: show ip ospf should identify Area 10 with the intended stub or NSSA behavior.
LSDB: Check show ip ospf database. Type 5 should be absent inside stub/NSSA; Type 7 should exist only for local NSSA externals.
Routes: Check the default, inter-area routes, and O N1/O N2 NSSA routes with show ip route ospf.
Traffic: Test both forward and return paths. An LSDB that looks correct does not prove end-to-end reachability.

Symptom	Likely cause	Best next check
Neighbor remains Down after area change	Stub/NSSA flag mismatch	Compare the area command on every router
No default in regular NSSA	Default was not requested from ABR	Check `default-information-originate`
No Type 7 for local static route	Redistribution or route installation failed	Check static route, redistribution, and NSSA status
Type 7 exists but no Type 5 outside	Translation issue	Check ABR translator state and P-bit
Routes vanish after no-summary	Specific Type 3 summaries were intentionally suppressed	Confirm the default route is sufficient

9. Guided Practice Lab

Keep one baseline capture and compare every design against it. Change one area feature at a time.

Task 1: Standard Area to Stub

Build Area 0 and Area 10 from the topology.
Redistribute 203.0.113.0/24 from R1.
Capture R3's LSDB and routes.
Configure area 10 stub on R2 and R3.
Prove Type 5 disappears and a default appears.

Expected result: R3 keeps inter-area routes but replaces external detail with a default route.

Task 2: Reduce Inter-Area Detail

Save R3's current O IA routes.
Add no-summary on the ABR only.
Confirm the adjacency stays Full.
Compare the new route table.
Test branch traffic through the default route.

Expected result: Most specific inter-area routes disappear while the Type 3 default remains.

Task 3: Build and Prove an NSSA

Return Area 10 to a normal area, then configure NSSA on both routers.
Create and redistribute 172.20.50.0/24 on R3.
Find its Type 7 LSA inside Area 10.
Find the translated Type 5 outside Area 10.
Verify default and end-to-end forwarding.

Expected result: You can trace one external route from Type 7 origin to Type 5 translation.

10. Frequently Asked Questions

What is the main purpose of an OSPF stub area?

It reduces external route detail inside an edge area. The ABR blocks Type 4 and Type 5 LSAs and provides a default route toward the rest of the OSPF domain.

What is the difference between stub and totally stubby areas?

A stub area still receives specific inter-area Type 3 summaries. A totally stubby area suppresses most of them and keeps a Type 3 default route.

When should I use an NSSA?

Use NSSA when the area should block external Type 5 LSAs from elsewhere but a router inside the area must redistribute a local external route as Type 7.

Must every router use the same stub or NSSA area type?

Yes. The area options in Hello packets must agree. A mismatch prevents the adjacency from reaching Full.

Does an NSSA receive a default route automatically?

A regular NSSA does not automatically receive one on Cisco IOS. Add default-information-originate on the ABR when needed. A totally NSSA receives a Type 3 default route through no-summary behavior.

OSPF Stub Areas and NSSA