CCIE OSPF LSA Areas

OSPF Area Types & Hierarchy

By Ioannis Alexander Konstas

Mastering OSPF area types is critical for the CCIE lab. It directly impacts your ability to perform LSA filtering, summarize routes, and optimize the routing domain. This article breaks down the rules, configuration, and design principles of OSPF areas.

Section 1

1. OSPF Router Roles

Before diving into area types, we must define the functional roles a router can play within an OSPF domain.

Internal Router: All of its interfaces belong to the same OSPF area. It only maintains a link-state database (LSDB) for that specific area.
ABR (Area Border Router): A router that has interfaces in multiple OSPF areas (one of which must be Area 0, the backbone). It maintains a separate LSDB for each area it connects to and acts as the gateway for exchanging inter-area routing information (generating Type 3 LSAs).
ASBR (Autonomous System Boundary Router): A router that redistributes routes from an external routing protocol (Static, BGP, EIGRP, RIP) into the OSPF domain (generating Type 5 or Type 7 LSAs).

Section 2

2. Normal (Non-Stub) Area

A normal area, which includes the backbone area (Area 0), imposes no restrictions on LSAs.

Accepts ALL LSA types: Type 1 (Router), Type 2 (Network), Type 3 (Summary), Type 4 (ASBR Summary), and Type 5 (External).
Provides full visibility of intra-area, inter-area, and external routes.
Think: "Give me everything."

Section 3

3. Stub Area

A stub area is designed to reduce the size of the routing table by blocking external routes.

Blocks Type 4 and Type 5 LSAs.
The ABR injects a default route (0.0.0.0/0) into the stub area using a Type 3 LSA, allowing internal routers to reach external destinations.
Still receives Type 1, 2, and 3 LSAs (inter-area routes).
CANNOT contain an ASBR. This is the golden rule of stub areas.
Think: "Don't send me external routes."

Stub Area Configuration (Must be on all routers in Area 1)

router ospf 1
 area 1 stub

Section 4

4. Totally Stubby Area

A Cisco-proprietary extension to the standard Stub area that further reduces LSDB and routing table size.

Blocks Type 3, 4, and 5 LSAs (except for the default route).
Only receives Type 1, 2, plus the default route (Type 3) injected by the ABR.
Produces the smallest possible routing table for internal routers.
Think: "Just give me a default route."

Totally Stubby ABR Configuration

router ospf 1
 area 1 stub no-summary

Exam Tip: You only need to configure no-summary on the ABR(s). Internal routers still just use the standard area 1 stub command. The ABR takes care of filtering the Type 3 LSAs.

Section 5

5. NSSA (Not-So-Stubby Area)

Introduced to overcome the "No ASBR in a Stub area" rule.

Used when you need stub behavior (blocking incoming external routes) BUT you also have an ASBR inside the area that needs to redistribute local external routes.
Blocks incoming Type 5 LSAs from the rest of the OSPF domain.
Allows redistribution inside the area by creating a special Type 7 LSA.
The NSSA ABR translates Type 7 LSAs into Type 5 LSAs when advertising them into Area 0.
Think: "I want stub behavior but I still need redistribution."

NSSA Configuration

router ospf 1
 area 1 nssa

Section 6

6. Totally NSSA

Combines NSSA and Totally Stubby features. This is the most restrictive area type that still permits local redistribution.

Allows Type 1, 2, local Type 7, and a default route (Type 3).
Blocks incoming Type 3, 4, and 5 LSAs.
Configured on the ABR with area 1 nssa no-summary.
Think: "Stub + NSSA combined."

Section 7

7. The Golden Rule

"Is there an ASBR inside the area?"

Yes → Cannot be Stub or Totally Stubby. You MUST use Normal or NSSA.
No → Stub or Totally Stubby is possible.

A stub area is NOT determined by topology alone — it MUST be explicitly configured. OSPF hello packets include an options field that indicates stub area capability; routers will not form adjacencies if they disagree on the stub flag.

Section 8

8. Common Misconception

Misconception: Many network engineers think that any remote site area connected via a single ABR to Area 0 (without an ASBR) is automatically acting as a stub area.

Truth: This is FALSE. Regardless of topology, unless explicitly configured with area X stub, the area remains a Normal area, and the ABR will flood all Type 3, 4, and 5 LSAs into it, wasting memory and CPU cycles on those remote routers.

Section 9

9. Decision Tree

Does area contain an ASBR?

Yes

Normal or NSSA

Want external routes?

Yes

Normal

Stub or
Totally Stubby

Section 10

10. Comprehensive Comparison Table

Area Type	Type 1,2	Type 3	Type 4	Type 5	Type 7	ASBR Allowed	Default Route
Normal	Yes	Yes	Yes	Yes	No	Yes	Manual Config
Stub	Yes	Yes	No	No	No	No	Injected by ABR
Totally Stubby	Yes	No*	No	No	No	No	Injected by ABR
NSSA	Yes	Yes	No	No	Yes	Yes	Optional Config
Totally NSSA	Yes	No*	No	No	Yes	Yes	Injected by ABR

* Only a single Type 3 LSA is allowed, which carries the default route (0.0.0.0/0).

Section 11

11. LSA Quick Reference

Type 1: Router LSA — Generated by all routers for their interfaces in a specific area.
Type 2: Network LSA — Generated by the DR on multi-access networks.
Type 3: Summary LSA — Generated by ABRs to advertise inter-area routes.
Type 4: ASBR Summary LSA — Generated by ABRs to provide reachability to an ASBR in another area.
Type 5: External LSA — Generated by ASBRs for routes redistributed into OSPF. Flooded domain-wide.
Type 7: NSSA External LSA — Generated by ASBRs inside an NSSA. Converted to Type 5 by the NSSA ABR.

Section 12

12. Memory Tricks

Progressive restriction: Normal → Stub → Totally Stubby
NSSA = A Stub that can redistribute locally.
Mantra: "Normal = Everything, Stub = No externals, Totally Stubby = Only default, NSSA = Stub with ASBR"

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