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)
