1.10 Troubleshoot OSPF (v2/v3)
1.10.c Network types, area types, and router types
📘CCNP Enterprise – ENARSI (300-410)
In OSPF, an area is a logical grouping of routers and networks. Areas help scale OSPF, reduce routing table size, and limit the scope of LSA flooding.
There are several area types, and each behaves differently in terms of routing updates and LSA types.
1. Backbone Area (Area 0)
- Purpose: The backbone is the core area in OSPF. All other areas must connect to it (directly or via a virtual link).
- OSPF Behavior:
- Connects all other areas.
- Carries inter-area routing information (LSAs that describe networks in other areas).
- Important LSAs in Backbone:
- Type 1 (Router LSA)
- Type 2 (Network LSA)
- Type 3 (Summary LSA) for networks in other areas
- Exam Tip:
- Always Area 0 – you cannot make another area the backbone.
- All non-backbone areas must connect to it.
2. Normal Area
- Purpose: A standard OSPF area that can carry both intra-area and inter-area routes.
- OSPF Behavior:
- Has full routing tables (all routes within the area + inter-area routes).
- Can contain internal routers and Area Border Routers (ABRs).
- Floods all LSA types: 1, 2, 3, and 4.
- Exam Tip:
- Normal areas are flexible and can have multiple ABRs.
- Good default choice for medium-sized OSPF deployments.
3. Transit Area
- Purpose: Special type of normal area used only to carry traffic between other areas.
- OSPF Behavior:
- Doesn’t have any internal hosts; only routers for connecting other areas.
- Type 3 LSAs (Summary LSAs) are forwarded through transit areas.
- Exam Tip:
- Rarely used in small networks, mostly in large enterprise or ISP networks.
- Must be a normal area, not stub or NSSA.
4. Stub Area
- Purpose: Designed to reduce routing table size and simplify routing for areas that don’t need external routes.
- OSPF Behavior:
- Blocks Type 5 LSAs (external routes from other OSPF domains or redistributed routes).
- Uses a default route (0.0.0.0) to reach external destinations.
- Floods only Type 1, 2, and 3 LSAs.
- Exam Tip:
- Ideal for branch offices with limited resources.
- Cannot have ASBR inside a stub area.
5. Totally Stubby Area
- Purpose: A stricter version of a stub area for maximum routing table reduction.
- OSPF Behavior:
- Blocks Type 3 (inter-area) and Type 5 (external) LSAs.
- Only allows a single default route from the ABR.
- Exam Tip:
- Useful when devices don’t need detailed routing info outside the area.
- Cisco-specific; configured using
area x stub no-summary.
6. Not-So-Stubby Area (NSSA)
- Purpose: A stub area that can import external routes (Type 7 LSAs) but still limits external flooding.
- OSPF Behavior:
- Blocks Type 5 LSAs from the OSPF domain.
- Allows Type 7 LSAs for locally redistributed external routes.
- ABR converts Type 7 → Type 5 when sending to other areas.
- Exam Tip:
- Used for branch offices that need to redistribute external routes but still want a smaller routing table.
- Can be configured as stub or totally stubby NSSA.
Quick Comparison Table
| Area Type | External LSAs | Inter-Area LSAs | Use Case/Note |
|---|---|---|---|
| Backbone (0) | Yes | Yes | Must connect all areas |
| Normal | Yes | Yes | Default area type |
| Transit | Yes | Yes | Only for routing between areas |
| Stub | No | Yes | Uses default route for external traffic |
| Totally Stubby | No | No | Uses single default route |
| NSSA | Limited (Type 7) | Yes | Stub + can import external routes |
Key Points for the Exam
- Backbone is mandatory (Area 0) – all other areas must connect.
- Stub areas reduce routing table size by blocking external LSAs.
- Totally stubby areas block both external and inter-area LSAs, only default route exists.
- NSSA allows external route redistribution in a mostly stubbed area.
- Transit areas are normal areas used only to forward traffic between other areas.
- LSAs are the main mechanism by which areas exchange routing info. Knowing which LSAs are allowed in which area is critical.
This structure is exam-focused, uses IT-relevant examples (branch office, ABR, ASBR), and makes complex concepts easier for non-IT readers.
