Common OSPF Passive Interface Mistakes

Mistake 2: Forgetting to Make User VLANs Passive

Problem: User-facing VLANs send OSPF Hellos

Impact:

• Security risk (rogue routers can join)
• Wasted resources

Fix:

router ospf 1
 passive-interface vlan 10
 passive-interface vlan 20

Or use default:

router ospf 1
 passive-interface default
 no passive-interface gi0/0  ← Uplink only

Mistake 3: Using passive-default Without Exclusions

Problem:

router ospf 1
 passive-interface default
 ! (Forgot to exclude uplinks)

Impact: ALL interfaces passive, no neighbors

Fix:

router ospf 1
 no passive-interface gi0/0
 no passive-interface gi0/1

Verification

R1# show ip protocols | include Passive
  Passive Interface(s):
    Vlan10
    Vlan20
    Loopback0

Should see: User VLANs, loopbacks
Should NOT see: Uplinks to other OSPF routers

Summary

✅ Never make uplinks passive
✅ Always make user VLANs passive
✅ Always make loopbacks passive
✅ Use passive-default on edge routers

Next: Subnet Mask Mismatch (Article 25)

Internal Links:

• ← Routes Not Appearing (Article 23)
• ← OSPF Passive Interfaces (Article 9)
• ← Neighbors Not Forming (Article 18)