OSPF Subnet Mask Mismatch: How to Troubleshoot and Fix

Detection

Check Subnet Masks

R1# show ip interface gi0/0 | include address
  Internet address is 10.1.1.1/30

R2# show ip interface gi0/0 | include address
  Internet address is 10.1.1.2/24  ← MISMATCH!

Neighbor State

R1# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address
2.2.2.2         1     INIT/DROTHER    00:00:35    10.1.1.2

INIT = One-way communication (likely subnet mismatch)

Fix

Match subnet masks:

R2(config)# interface gi0/0
R2(config-if)# ip address 10.1.1.2 255.255.255.252

Verify:

R1# show ip ospf neighbor
Neighbor ID     Pri   State           Dead Time   Address
2.2.2.2         1     FULL/DR         00:00:38    10.1.1.2

✅ FULL state = Fixed!

Summary

✅ Broadcast networks require matching masks
✅ Symptom — Stuck at Init/2-Way
✅ Fix — Match subnet masks

Next: SPF Algorithm (Article 27)

Internal Links:

• ← Passive Interface Mistakes (Article 24)
• ← Neighbors Not Forming (Article 18)
• → SPF Algorithm (Article 27)