Common Causes
1. Better Administrative Distance
Problem: Another routing protocol has better AD
Example:
- EIGRP (AD 90) beats OSPF (AD 110)
Check:
R1# show ip route 192.168.10.0
D 192.168.10.0/24 [90/2816] via 10.1.1.2 ← EIGRP won
Solution: Remove other protocol or adjust AD
2. Passive Interface
Problem: Interface advertising route is passive (can't be next-hop)
Check:
R1# show ip protocols | include Passive
Passive Interface(s):
GigabitEthernet0/1 ← Next-hop interface is passive
Solution: Remove passive if interface needs to forward
3. Route Filtering
Problem: Distribute-list blocking route
Check:
R1# show ip protocols | include distribute-list
Distribute list 10 in from 2.2.2.2
Solution: Check ACL 10, remove filter if incorrect
4. Inactive Next-Hop
Problem: Next-hop IP not reachable
Check routing table for next-hop
Solution: Fix next-hop reachability
Verification Steps
1. Confirm Route in LSDB
R1# show ip ospf database | include 192.168.10.0
2. Check for Better Route
R1# show ip route 192.168.10.0
3. Check OSPF SPF Log
R1# show ip ospf | include SPF
Summary
✅ Check LSDB — Route should be there
✅ Check AD — Other protocol may win
✅ Check passive — Interface may be passive
✅ Check filtering — Distribute-list may block
Next: Passive Interface Mistakes (Article 24)
Internal Links:
- ← Authentication Mismatch (Article 21)
- ← OSPF Cost (Article 6)
- → Passive Interface Mistakes (Article 24)
