Lab 14: Multi-Area OSPF with an ABR
Scale OSPF the real way: R1 in area 0, R3 in area 1, and R2 as the ABR in both — then watch inter-area routes appear as O IA. Difficulty: Advanced · Time: ~40 min.
Lab objectives
- Build a two-area OSPF (0 and 1) with R2 as ABR
- Assign the right interfaces to the right areas
- Identify O vs O IA routes
- See the ABR in show ip ospf
Topology & addressing
R1—R2 link 10.0.12.0/30 in area 0 (R1 LAN 192.168.1.0/24 also area 0). R2—R3 link 10.0.23.0/30 in area 1 (R3 LAN 192.168.3.0/24 area 1). R2 = ABR.
Step-by-step configuration
R1: router ospf 1network 192.168.1.0 0.0.0.255 area 0network 10.0.12.0 0.0.0.3 area 0 | Pure area 0 router |
R2: network 10.0.12.0 0.0.0.3 area 0network 10.0.23.0 0.0.0.3 area 1 | Feet in both areas = ABR |
R3: network 10.0.23.0 0.0.0.3 area 1network 192.168.3.0 0.0.0.255 area 1 | Pure area 1 router |
Verification
On R1, show ip route ospf: R3's LAN appears as O IA (inter-area, via the ABR) while same-area routes are plain O. show ip ospf on R2 declares "area border router". End-to-end ping R1-LAN → R3-LAN proves the hierarchy routes.
Next lab: labs hub · test yourself: CCNA practice test.
Frequently asked questions
What makes a router an ABR?
Having interfaces in area 0 AND another area — it then summarises and passes routes between them as Type 3 LSAs.
What does O IA mean in the routing table?
An OSPF inter-area route — learned from another area through an ABR, versus O for intra-area routes.
Why must every area touch area 0?
OSPF's loop-prevention design: inter-area traffic transits the backbone, so non-zero areas must attach to it (or be virtually linked).
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