Fixing OSPF Area Mismatch Issues

Detection

Check Area ID on Each Router

R1:

R1# show ip ospf interface gi0/0 | include Area
  Internet Address 10.1.1.1/30, Area 0

R2:

R2# show ip ospf interface gi0/0 | include Area
  Internet Address 10.1.1.2/30, Area 10  ← MISMATCH!

Area 0 ≠ Area 10 = No adjacency

Debug Output

R1# debug ip ospf adj
*OSPF-1 ADJ: Rcv pkt from 10.1.1.2, area 10 : src not on the same area

Fix

Change R2's area to match R1:

R2(config)# router ospf 1
R2(config-router)# no network 10.1.1.0 0.0.0.3 area 10
R2(config-router)# network 10.1.1.0 0.0.0.3 area 0

Verify:

R2# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1         1     FULL/  -        00:00:35    10.1.1.1        Gi0/0

✅ Fixed!

Summary

✅ Check area IDs with show ip ospf interface
✅ Areas must match on both sides
✅ Fix by changing network statement area

Next: Authentication Mismatch (Article 21)

Internal Links:

• ← OSPF Neighbors Not Forming (Article 18)
• → Authentication Mismatch (Article 21)