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
-
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. -
All ABRs must have at least one interface in Area 0
ABRs connect non-backbone areas to Area 0. -
Area 0 cannot be a stub area
The backbone must accept all LSA types. -
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:
- Multi-area topology diagram showing Area 0, ABRs, and branch areas
- Flowchart: Traffic from Area 10 → Area 0 → Area 20
show ip ospfoutput highlighting ABR and area info- 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)
