OSPF Areas: Area 0, ABRs, Stub and NSSA Design

Q: What does O IA mean in the routing table?

O IA means an OSPF inter-area route. The prefix originated in another area and was advertised into the local area by an ABR.

Q: Do OSPF area numbers need to match on both ends of a link?

Yes. The interfaces on a shared OSPF link must use the same area ID. A mismatch prevents a neighbor relationship from forming.

OSPF Areas at a Glance

An OSPF area is a group of routers and links that share detailed topology information. Area 0 connects the other areas. An Area Border Router, or ABR, joins Area 0 to a non-backbone area.

BackboneArea 0

Boundary routerABR

Remote-area routeO IA

Main benefitSmaller LSDB

Choose your track: Read sections 1–3 to understand the design. Continue through sections 4–8 to configure, verify, break, and repair a multi-area lab.

1. See the Multi-Area Design

In this lesson, Area 10 is a branch, Area 0 is the backbone, and Area 20 is another branch. R2 connects Area 10 to Area 0. R4 connects Area 0 to Area 20. That makes R2 and R4 ABRs.

Area 10 · Branch

R1Internal router

R2ABR

10.12.0.0/30 · 10.1.1.1/32

Area 0 · Backbone

R2ABR

R3Backbone

R4ABR

10.23.0.0/30 · 10.34.0.0/30

Area 20 · Branch

R4ABR

R5Internal router

10.45.0.0/30 · 10.5.5.5/32

R2 and R4 appear in both areas they connect because each ABR keeps a separate LSDB for every attached area. Inter-area traffic crosses Area 0.

Inside one area: Routers share detailed Type 1 and Type 2 topology information.
At the boundary: The ABR describes networks from one area to another with Type 3 summary LSAs.
In the routing table: A route learned from another area normally appears as O IA.

2. Understand ABRs and Inter-Area Routes

An ABR has active OSPF interfaces in Area 0 and at least one other area. It does not copy the complete topology from one area into another. Instead, it advertises reachability between areas.

What you inspect	Inside the local area	From another area
Topology detail	Type 1 and Type 2 LSAs	Type 3 summary LSAs from an ABR
Route code	`O` for intra-area	`O IA` for inter-area
SPF calculation	Full local-area topology	Reachability through the ABR

Important: “Summary LSA” does not always mean that several prefixes were combined. A Type 3 LSA can describe one specific inter-area prefix. Route summarization is a separate configuration choice.

3. Compare OSPF Area Types

Choose an area type by asking one question: how much external and inter-area detail does this area need? A default route can replace detail at a simple branch.

Area type	Key behavior	Good fit
Backbone	Area 0 carries inter-area reachability between non-backbone areas.	The stable core of a multi-area design.
Normal	Accepts normal inter-area and external route information.	An area that needs complete routing detail.
Stub	Blocks Type 5 external LSAs and receives a default route from the ABR.	A branch with no local redistribution.
Totally stubby	Also suppresses most Type 3 detail; the ABR provides a default route. This is a Cisco extension.	A simple branch that needs very little route detail.
NSSA	Blocks Type 5 LSAs but allows a local ASBR to advertise external routes as Type 7 LSAs.	A branch that must redistribute one local route source.
Totally NSSA	Combines NSSA redistribution with stronger inter-area suppression and a default route.	A simple redistribution branch.

4. Configure a Three-Area Topology

The area number is applied to an interface. Both ends of an OSPF link must agree on the area. These examples configure the two ABRs in the topology.

R2 · Area 10 to Area 0

R2# configure terminal
R2(config)# router ospf 1
R2(config-router)# router-id 2.2.2.2
R2(config-router)# network 10.12.0.0 0.0.0.3 area 10
R2(config-router)# network 10.23.0.0 0.0.0.3 area 0

R4 · Area 0 to Area 20

R4# configure terminal
R4(config)# router ospf 1
R4(config-router)# router-id 4.4.4.4
R4(config-router)# network 10.34.0.0 0.0.0.3 area 0
R4(config-router)# network 10.45.0.0 0.0.0.3 area 20

Process IDs are local: The number after router ospf does not need to match between routers. The area assigned to the shared link does need to match.

5. Configure Stub and NSSA Behavior

Every router inside the area must agree that the area is stub or NSSA. The no-summary option is configured only on the ABR when you want the totally stubby or totally NSSA behavior.

Make Area 20 stub

! Configure on R4 (ABR) and R5
router ospf 1
 area 20 stub

! For totally stubby, use on R4 only
 area 20 stub no-summary

Make Area 20 NSSA

! Configure on R4 (ABR) and R5
router ospf 1
 area 20 nssa

! For totally NSSA, use on R4 only
 area 20 nssa no-summary

Stub area: Use it only when the area does not contain an ASBR that must redistribute routes.
NSSA: Use it when the area should remain stub-like but still needs local redistribution.
Default route: Confirm that internal routers still have a path out of the reduced-detail area.

6. Verify Areas, ABRs, and Routes

Verify the design from three views: interface membership, ABR status, and route type. Do not rely on a successful ping alone.

Check the local OSPF role

R2# show ip ospf
 Routing Process "ospf 1" with ID 2.2.2.2
 It is an area border router
 Number of areas in this router is 2
 Area BACKBONE(0)
 Area 10

Check a remote-area route

R1# show ip route ospf
O IA 10.5.5.5/32 [110/4] via 10.12.0.2

R1# show ip ospf database summary
Summary Net Link States (Area 10)

Command	What it proves
`show ip ospf interface brief`	Each interface is in the intended area.
`show ip ospf border-routers`	The router knows paths to ABRs and ASBRs.
`show ip ospf database summary`	Type 3 inter-area reachability exists.
`show ip route ospf`	Remote-area prefixes are installed as `O IA`.

7. Troubleshoot Area Problems

Start at the failed link and move outward. An area problem usually appears before the missing route problem.

Check the neighbor. If it is down, compare the area ID on both ends of the link first.
Check Area 0 continuity. A non-backbone area needs a working path to the backbone for normal inter-area routing.
Check the ABR role. The router must have at least one active interface in Area 0 and another active interface in a different area.
Check area flags. A stub or NSSA mismatch prevents neighbors from reaching Full state.
Check Type 3 LSAs before the route table. If the summary LSA is missing, the ABR did not advertise the prefix into the area.

8. Guided Practice Lab

Build the five-router topology shown above. Complete one task at a time and save the verification output before changing the design.

Task 1: Build Working Inter-Area Routing

Configure Area 10, Area 0, and Area 20 as shown.
Advertise loopback 10.1.1.1/32 from R1 and 10.5.5.5/32 from R5.
Confirm that R2 and R4 report themselves as ABRs.
Check the R5 loopback route on R1.

Expected result: R1 learns 10.5.5.5/32 as an O IA route through R2.

Task 2: Create and Repair an Area Mismatch

In the isolated lab, move R4's R3-facing interface from Area 0 to Area 30.
Observe the R3–R4 neighbor failure and the missing remote routes.
Compare show ip ospf interface brief on R3 and R4.
Restore Area 0 and verify the neighbor and routes.

Expected result: You can identify the mismatched area on the shared link and prove that the repair restored inter-area routing.

Task 3: Turn Area 20 into a Stub Area

Create one test external route outside Area 20 and confirm that R5 sees it.
Configure area 20 stub on R4 and R5.
Compare R5's LSDB and routing table with the baseline.
Confirm that a default route provides reachability out of Area 20.

Expected result: Type 5 detail is removed from Area 20, while R5 keeps external reachability through the ABR's default route.

9. Frequently Asked Questions

Why must non-backbone areas connect to Area 0?

Area 0 is the transit backbone for normal inter-area routing. ABRs exchange reachability between non-backbone areas through the backbone.

What makes a router an OSPF ABR?

An ABR has active OSPF interfaces in Area 0 and at least one other area. It keeps a separate LSDB for every attached area.

What does O IA mean in the routing table?

O IA means an OSPF inter-area route. The prefix originated in another area and was advertised into the local area by an ABR.

What is the difference between a stub area and an NSSA?

Both block Type 5 external LSAs. An NSSA also allows a local ASBR to redistribute routes as Type 7 LSAs. A standard stub area does not allow that local redistribution role.

Do OSPF area numbers need to match on both ends of a link?

Yes. The two interfaces on a shared OSPF link must use the same area ID. A mismatch prevents a neighbor relationship from forming.

OSPF Areas: Backbone, Normal, Stub and NSSA Design