OSPF Areas Explained: Why and How to Use Them

The Problem OSPF Areas Solve

Scalability Issues Without Areas

Imagine a flat OSPF network (all routers in one area) with 200 routers:

❌ Problem 1: Large LSDB
Every router stores LSAs from all 200 routers. LSDB size = huge.

❌ Problem 2: SPF Overhead
When a single link goes down, all 200 routers re-run SPF. CPU spike across the entire network.

❌ Problem 3: LSA Flooding
When a link changes state, the LSA floods to all 200 routers.

How Areas Fix This

With areas, you divide the 200 routers into smaller groups:

✅ Smaller LSDBs
Routers only need detailed LSAs about their own area.

✅ Isolated SPF
A link failure in Area 10 only triggers SPF in Area 10, not in Areas 20, 30, or 0.

✅ Reduced Flooding
LSAs are summarized at area boundaries instead of flooding everywhere.

Area 0: The Backbone Area

Area 0 (also called Area 0.0.0.0 or the backbone area) is special. It's the central hub through which all inter-area traffic flows.

Key Rules About Area 0

1. All inter-area traffic must pass through Area 0
   If a router in Area 10 wants to reach a network in Area 20, the path goes: Area 10 → Area 0 → Area 20.
2. All ABRs must have at least one interface in Area 0
   ABRs connect non-backbone areas to Area 0.
3. Area 0 cannot be a stub area
   The backbone must accept all LSA types.
4. Area 0 must be contiguous
   You can't have Area 0 split into disconnected pieces. If Area 0 is broken, you need a virtual link.

Think of Area 0 as a highway system:

• Non-backbone areas are cities
• Area 0 is the interstate highway connecting them
• ABRs are the on-ramps

Area Types

Standard Area (Area 0 or Non-Backbone)

What it is:
A normal OSPF area that accepts all LSA types.

Where it's used:

• Area 0 (always standard)
• Non-backbone areas in smaller networks

LSAs allowed:
All types (1, 2, 3, 4, 5, 7)

Stub Area

What it is:
An area that blocks external LSAs (Type 5). Instead, the ABR injects a default route.

Why you'd use it:
Branch offices that don't need to know about external routes (routes from other protocols or static routes redistributed into OSPF).

LSAs allowed:
Types 1, 2, 3 (no Type 4 or 5)

Configuration:

Router(config-router)# area 10 stub

Learn more: OSPF Stub Area Configuration (Article 14)

Totally Stubby Area (Cisco Proprietary)

What it is:
Like a stub area, but also blocks inter-area summary LSAs (Type 3). The only Type 3 LSA allowed is the default route.

Why you'd use it:
Maximum route reduction in small branch offices.

LSAs allowed:
Types 1, 2, and one Type 3 (default route only)

Configuration (on ABR):

Router(config-router)# area 10 stub no-summary

Learn more: OSPF Stub Area Configuration (Article 14)

Not-So-Stubby Area (NSSA)

What it is:
A stub area that allows limited external route injection via Type 7 LSAs.

Why you'd use it:
You want the benefits of a stub area, but you have a router redistributing external routes (e.g., a branch office router redistributing a static route).

LSAs allowed:
Types 1, 2, 3, 7 (Type 7 is converted to Type 5 at the ABR)

Configuration:

Router(config-router)# area 10 nssa

Learn more: OSPF Stub Area Configuration (Article 14)

Totally NSSA (Cisco Proprietary)

What it is:
Combines totally stubby and NSSA.

LSAs allowed:
Types 1, 2, 7, and one Type 3 (default route only)

Configuration (on ABR):

Router(config-router)# area 10 nssa no-summary

Router Types in a Multi-Area OSPF Network

Internal Router

What it is:
A router with all interfaces in the same area.

Example:
All interfaces in Area 10.

What it knows:

• Detailed topology of its own area (Type 1 and Type 2 LSAs)
• Summarized routes to other areas (Type 3 LSAs)
• External routes (Type 5 LSAs, if not in a stub area)

Area Border Router (ABR)

What it is:
A router with interfaces in multiple areas. At least one interface must be in Area 0.

Example:

• Interface Gi0/0 in Area 0
• Interface Gi0/1 in Area 10

What it does:

• Summarizes routes from one area and advertises them into other areas (Type 3 LSAs)
• Maintains separate LSDBs for each area
• Blocks Type 5 LSAs from entering stub areas

Where you'll see it:

Router# show ip ospf
  Routing Process "ospf 1" with ID 10.0.0.1
  ...
  Area BACKBONE(0)
    Number of interfaces in this area is 1
  Area 10
    Number of interfaces in this area is 1
  This router is an ABR

Learn more: Configuring Multi-Area OSPF (Article 12)

Backbone Router

What it is:
A router with at least one interface in Area 0.

Note:
All ABRs are backbone routers, but not all backbone routers are ABRs.

Autonomous System Boundary Router (ASBR)

What it is:
A router that injects external routes into OSPF (from static routes, RIP, EIGRP, BGP, etc.).

Example:
An edge router redistributing a static default route.

What it does:

• Generates Type 5 LSAs (or Type 7 in NSSAs)
• Advertises itself as an ASBR in Type 4 LSAs

Learn more: OSPF Redistribution (Article 29)

Single-Area vs Multi-Area OSPF

When to Use Single-Area OSPF

Use single-area OSPF when:

✅ Small to medium networks (up to ~50 routers)
✅ All routers are in the same location
✅ Convergence time is acceptable
✅ Simplicity is more important than optimization

Example:
A campus network with 20 routers, all in one building.

Configuration is simple:
All routers in Area 0. No ABRs, no summarization needed.

When to Use Multi-Area OSPF

Use multi-area OSPF when:

✅ Large networks (100+ routers)
✅ Geographically distributed (HQ + branches)
✅ You want to isolate SPF calculations
✅ You want to summarize routes
✅ You have stub areas (branches with simple connectivity)

Example:

• Area 0: Core/backbone routers
• Area 10: Branch office 1
• Area 20: Branch office 2
• Area 30: Data center

Learn more: Configuring Multi-Area OSPF (Article 12)

How LSAs Flow Between Areas

Within an Area (Intra-Area)

Routers exchange Type 1 (Router LSAs) and Type 2 (Network LSAs) within the area. Every router has the same detailed LSDB.

Example:
Router A and Router B are both in Area 10. They exchange Type 1 and Type 2 LSAs describing all links in Area 10.

Between Areas (Inter-Area)

ABRs summarize routes from one area and advertise them into another area as Type 3 (Summary LSAs).

Example:

• Area 10 has network 10.1.0.0/16
• ABR summarizes it and sends a Type 3 LSA to Area 0 advertising 10.1.0.0/16
• Routers in Area 0 know the network exists but don't know the detailed topology of Area 10

Key point:
Routers in Area 0 don't run SPF when a link flaps in Area 10—they only see a route update.

External Routes

ASBRs inject external routes as Type 5 LSAs (or Type 7 in NSSAs). Type 5 LSAs flood throughout the OSPF domain (except into stub areas).

Learn more: OSPF LSA Types Explained (Article 26)

Practical Multi-Area Design Example

Scenario:

• Headquarters with core routers
• 3 branch offices
• Each branch has simple connectivity (one link to HQ)

Design:

Area 0 (Backbone):

• HQ core routers
• Links between core routers

Area 10 (Branch 1):

• Branch 1 routers
• ABR: HQ edge router connecting to Branch 1

Area 20 (Branch 2):

• Branch 2 routers
• ABR: HQ edge router connecting to Branch 2

Area 30 (Branch 3):

• Branch 3 routers
• ABR: HQ edge router connecting to Branch 3
• Configured as stub area (no external routes needed)

Benefits:

• Branch failures don't trigger SPF in HQ
• Stub area reduces routing table size in Branch 3
• Summarization at ABRs reduces LSAs

Area Design Best Practices

1. Keep Area 0 Stable

Area 0 is critical. If it goes down, inter-area routing fails. Design Area 0 with:

• Redundant links
• Reliable hardware
• Minimal churn

2. Design Areas by Geography or Function

Good area design:

• Area 0: HQ core
• Area 10: East coast branches
• Area 20: West coast branches
• Area 30: Data center

Bad area design:

• Area 10: Routers 1-50 (arbitrary grouping)
• Area 20: Routers 51-100

3. Use Stub Areas for Simple Branches

If a branch office:

• Has one or two links to HQ
• Doesn't need external routes
• Has no ASBR

Make it a stub area (or totally stubby).

4. Summarize at Area Boundaries

Use area X range on ABRs to summarize networks.

Example:

Router(config-router)# area 10 range 10.10.0.0 255.255.0.0

This advertises a single 10.10.0.0/16 route into Area 0 instead of dozens of /24 routes.

Learn more: OSPF Route Summarization (Article 28)

5. Avoid Too Many Areas

More areas = more ABRs = more complexity.

Rule of thumb:

• Small networks: 1 area (Area 0)
• Medium networks: 3-5 areas
• Large networks: 10-20 areas (rarely more)

Common Area Design Mistakes

Mistake 1: Non-Backbone Areas Directly Connected

Problem:
Area 10 and Area 20 share a link, but neither is Area 0.

Result:
OSPF won't route between them. All inter-area traffic must pass through Area 0.

Fix:
Make the link between them part of Area 0, or use a virtual link (temporary workaround).

Learn more: OSPF Virtual Links (Article 15)

Mistake 2: Discontiguous Area 0

Problem:
Area 0 is split into two pieces (e.g., by Area 10).

Result:
Routing breaks. ABRs can't reach each other through Area 0.

Fix:
Reconfigure the areas so Area 0 is contiguous, or use a virtual link.

Mistake 3: Making Area 0 a Stub Area

Problem:
Someone configures area 0 stub.

Result:
OSPF won't work. Area 0 must accept all LSA types.

Verifying OSPF Areas

Show OSPF Process and Areas

Router# show ip ospf
 Routing Process "ospf 1" with ID 10.0.0.1
 Start time: 00:05:23.456, Time elapsed: 01:23:45.678
 Supports only single TOS(TOS0) routes
 This router is an ABR
 SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
 Area BACKBONE(0)
   Number of interfaces in this area is 2
   Area has no authentication
   SPF algorithm last executed 00:12:34.567 ago
 Area 10
   Number of interfaces in this area is 1
   Area has no authentication
   SPF algorithm last executed 00:02:15.123 ago

What you see:

• Router is an ABR (interfaces in multiple areas)
• Area 0 has 2 interfaces
• Area 10 has 1 interface
• SPF ran separately for each area

Show OSPF Interfaces per Area

Router# show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Gi0/0        1     0               10.0.0.1/30        1     P2P   1/1
Gi0/1        1     0               10.0.0.5/30        1     P2P   1/1
Gi0/2        1     10              10.10.1.1/24       1     DR    2/2

Show OSPF Database per Area

Router# show ip ospf database
            OSPF Router with ID (10.0.0.1)

                Router Link States (Area 0)
Link ID         ADV Router      Age         Seq#       Checksum Link count
10.0.0.1        10.0.0.1        345         0x80000005 0x00A1B2 2
10.0.0.2        10.0.0.2        123         0x80000003 0x00C3D4 3

                Summary Net Link States (Area 0)
Link ID         ADV Router      Age         Seq#       Checksum
10.10.0.0       10.0.0.1        456         0x80000002 0x00E5F6

                Router Link States (Area 10)
Link ID         ADV Router      Age         Seq#       Checksum Link count
10.0.0.1        10.0.0.1        234         0x80000004 0x001122 1
10.10.1.10      10.10.1.10      678         0x80000006 0x003344 2

What you see:

• Separate Router LSAs for Area 0 and Area 10
• Summary LSAs in Area 0 (from Area 10)

Summary: OSPF Area Checklist

Now you know:

✅ What areas are and why they exist (scalability)
✅ Area 0 is the backbone and all inter-area traffic flows through it
✅ ABRs connect areas and summarize routes
✅ Area types: Standard, stub, totally stubby, NSSA
✅ Single-area vs multi-area design considerations
✅ Common mistakes: Non-contiguous Area 0, direct area connections

Next Step:
Now that you understand areas conceptually, it's time to learn about DR and BDR—the routers that manage OSPF on multi-access networks. Read OSPF DR and BDR Explained next.

Screenshot Suggestions:

1. Multi-area topology diagram showing Area 0, ABRs, and branch areas
2. Flowchart: Traffic from Area 10 → Area 0 → Area 20
3. show ip ospf output highlighting ABR and area info
4. Side-by-side: Single-area vs multi-area LSDB size comparison

Internal Links:

• ← OSPF Key Terms and Concepts (Article 2)
• → OSPF DR and BDR Explained (Article 5)
• → Configuring Multi-Area OSPF (Article 12)
• → OSPF Stub Area Configuration (Article 14)
• → OSPF Virtual Links (Article 15)