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OSPF · · 6 min read

Configuring Multi-Area OSPF on Cisco Routers

Configuring Multi-Area OSPF on Cisco Routers

Multi-Area OSPF Rules

  1. Area 0 is the backbone — All inter-area traffic flows through it
  2. All ABRs must connect to Area 0 — At least one interface in Area 0
  3. Non-backbone areas connect to Area 0 — Not directly to each other
  4. Area 0 must be contiguous — Cannot be split

Lab Topology

        Area 0 (Backbone)
    [R1]----[R2]----[R3]
     |               |
   Area 10        Area 20
     |               |
    [R4]            [R5]

Design:

Networks:

Configuration

R1 (ABR: Area 0 + Area 10)

interface loopback 0
 ip address 1.1.1.1 255.255.255.255

interface gi0/0
 description Link to R2 - Area 0
 ip address 10.0.12.1 255.255.255.252

interface gi0/1
 description Link to R4 - Area 10
 ip address 172.16.14.1 255.255.255.252

router ospf 1
 router-id 1.1.1.1
 network 1.1.1.1 0.0.0.0 area 0
 network 10.0.12.0 0.0.0.3 area 0
 network 172.16.14.0 0.0.0.3 area 10

Key: Same router, different areas on different interfaces.

R2 (Backbone Router: Area 0 only)

interface loopback 0
 ip address 2.2.2.2 255.255.255.255

interface gi0/0
 description Link to R1 - Area 0
 ip address 10.0.12.2 255.255.255.252

interface gi0/1
 description Link to R3 - Area 0
 ip address 10.0.23.2 255.255.255.252

router ospf 1
 router-id 2.2.2.2
 network 2.2.2.2 0.0.0.0 area 0
 network 10.0.12.0 0.0.0.3 area 0
 network 10.0.23.0 0.0.0.3 area 0

R3 (ABR: Area 0 + Area 20)

interface loopback 0
 ip address 3.3.3.3 255.255.255.255

interface gi0/1
 description Link to R2 - Area 0
 ip address 10.0.23.3 255.255.255.252

interface gi0/2
 description Link to R5 - Area 20
 ip address 192.168.35.3 255.255.255.252

router ospf 1
 router-id 3.3.3.3
 network 3.3.3.3 0.0.0.0 area 0
 network 10.0.23.0 0.0.0.3 area 0
 network 192.168.35.0 0.0.0.3 area 20

R4 (Internal Router: Area 10)

interface loopback 0
 ip address 4.4.4.4 255.255.255.255

interface loopback 1
 ip address 172.16.100.1 255.255.255.0
 description User Network

interface gi0/1
 description Link to R1 - Area 10
 ip address 172.16.14.4 255.255.255.252

router ospf 1
 router-id 4.4.4.4
 network 4.4.4.4 0.0.0.0 area 10
 network 172.16.0.0 0.0.255.255 area 10

R5 (Internal Router: Area 20)

interface loopback 0
 ip address 5.5.5.5 255.255.255.255

interface loopback 1
 ip address 192.168.200.1 255.255.255.0
 description User Network

interface gi0/2
 description Link to R3 - Area 20
 ip address 192.168.35.5 255.255.255.252

router ospf 1
 router-id 5.5.5.5
 network 5.5.5.5 0.0.0.0 area 20
 network 192.168.0.0 0.0.255.255 area 20

Verification

Check OSPF Process and Areas

R1 (ABR):

R1# show ip ospf
 Routing Process "ospf 1" with ID 1.1.1.1
 Supports opaque LSA
 This router is an ABR
 Area BACKBONE(0)
   Number of interfaces in this area is 2
   SPF algorithm last executed 00:05:23.456 ago
 Area 10
   Number of interfaces in this area is 1
   SPF algorithm last executed 00:02:15.123 ago

Key: "This router is an ABR" + two areas listed

Check OSPF Neighbors

R1:

R1# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2         1     FULL/  -        00:00:35    10.0.12.2       Gi0/0
4.4.4.4         1     FULL/  -        00:00:38    172.16.14.4     Gi0/1

R2 (backbone only):

R2# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1         1     FULL/  -        00:00:36    10.0.12.1       Gi0/0
3.3.3.3         1     FULL/  -        00:00:34    10.0.23.3       Gi0/1

Check Routing Table

R4 (Area 10):

R4# show ip route ospf
     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/2] via 172.16.14.1, Gi0/1
     2.0.0.0/32 is subnetted, 1 subnets
O IA    2.2.2.2 [110/3] via 172.16.14.1, Gi0/1
     3.0.0.0/32 is subnetted, 1 subnets
O IA    3.3.3.3 [110/4] via 172.16.14.1, Gi0/1
     5.0.0.0/32 is subnetted, 1 subnets
O IA    5.5.5.5 [110/5] via 172.16.14.1, Gi0/1
     10.0.0.0/8 is variably subnetted
O IA    10.0.12.0/30 [110/2] via 172.16.14.1, Gi0/1
O IA    10.0.23.0/30 [110/3] via 172.16.14.1, Gi0/1
O IA 192.168.35.0/30 [110/4] via 172.16.14.1, Gi0/1
O IA 192.168.200.0/24 [110/5] via 172.16.14.1, Gi0/1

Key indicators:

Test Connectivity

R4 to R5 (Area 10 → Area 20):

R4# ping 5.5.5.5 source 4.4.4.4
!!!!!
Success rate is 100 percent (5/5)

R4# ping 192.168.200.1 source 172.16.100.1
!!!!!
Success rate is 100 percent (5/5)

✅ Full inter-area connectivity!

Check OSPF Database

R1 (ABR):

R1# show ip ospf database

            OSPF Router with ID (1.1.1.1)

                Router Link States (Area 0)
Link ID         ADV Router
1.1.1.1         1.1.1.1
2.2.2.2         2.2.2.2
3.3.3.3         3.3.3.3

                Summary Net Link States (Area 0)
Link ID         ADV Router
172.16.14.0     1.1.1.1  ← R1 advertising Area 10 into Area 0
172.16.100.0    1.1.1.1
192.168.35.0    3.3.3.3  ← R3 advertising Area 20 into Area 0
192.168.200.0   3.3.3.3

                Router Link States (Area 10)
Link ID         ADV Router
1.1.1.1         1.1.1.1
4.4.4.4         4.4.4.4

                Summary Net Link States (Area 10)
Link ID         ADV Router
10.0.12.0       1.1.1.1  ← R1 advertising Area 0 into Area 10
10.0.23.0       1.1.1.1
192.168.35.0    1.1.1.1  ← R1 advertising Area 20 into Area 10
192.168.200.0   1.1.1.1

Key: ABRs generate Summary (Type 3) LSAs between areas.

Inter-Area Routing Flow

Path from R4 (Area 10) to R5 (Area 20):

  1. R4 sends traffic to R1 (ABR)
  2. R1 forwards to Area 0 (R2)
  3. R2 forwards to R3 (ABR)
  4. R3 forwards to R5 (Area 20)

All inter-area traffic flows through Area 0.

Common Multi-Area Mistakes

Mistake 1: Non-Backbone Areas Connected Directly

Bad design:

R4 (Area 10) ---- R5 (Area 20)  ← Direct link

Problem: OSPF won't route between them. Must go through Area 0.

Fix: Make the direct link part of Area 0, or route via Area 0.

Mistake 2: ABR Not in Area 0

Bad config:

R1:
  Gi0/0 in Area 10
  Gi0/1 in Area 20
  (No interface in Area 0!)

Problem: R1 can't be an ABR. All ABRs need at least one Area 0 interface.

Fix: Ensure ABR has an interface in Area 0.

Mistake 3: Discontiguous Area 0

Bad design:

R1 (Area 0) ---- R2 (Area 10) ---- R3 (Area 0)

Area 0 is split by Area 10.

Problem: ABRs can't reach each other through Area 0.

Fix: Use virtual link (temporary) or redesign topology.

Route Summarization at ABRs

ABRs can summarize routes between areas:

R1 (summarize Area 10 networks):

router ospf 1
 area 10 range 172.16.0.0 255.255.0.0

Result: R1 advertises a single 172.16.0.0/16 into Area 0 instead of individual subnets.

Learn more: OSPF Route Summarization (Article 28)

Best Practices

1. Design Area 0 with Redundancy

Area 0 is critical. Use redundant links and routers.

2. Keep Internal Routers Simple

Internal routers (like R4, R5) only see their area + summaries. This is good—keep them simple.

3. Place ABRs Strategically

ABRs are high-traffic routers (inter-area forwarding). Use powerful hardware.

4. Document Area Assignments

Maintain a diagram showing which routers/links are in which areas.

5. Use Summarization

Reduce routing table size by summarizing at ABR boundaries.

Summary

Now you know:

✅ Why multi-area OSPF — Scalability, SPF isolation
✅ Multi-area rules — Area 0 backbone, ABRs connect to Area 0
✅ How to configure — Different areas on different interfaces
✅ How to verify — Check ABR status, inter-area routes (O IA)
✅ Common mistakes — Direct area connections, missing Area 0
✅ Best practices — Redundant Area 0, strategic ABR placement

Next Step:
Multi-area OSPF needs security. Learn How to Configure OSPF Authentication next.

Screenshot Suggestions:

  1. Multi-area topology diagram with areas labeled
  2. show ip ospf on ABR highlighting areas
  3. show ip route ospf showing O IA routes
  4. show ip ospf database showing Summary LSAs

Internal Links:

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