OSPF Troubleshooting !!!!

Step 1: Verify Basic Connectivity

✅ ping <neighbor IP> → Ensure devices can reach each other.
✅ show interfaces status → Check interface status (UP/Down).
✅ show ip arp → Verify correct MAC resolution.

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Step 2: Check OSPF Neighbor Relationship

✅ show ip ospf neighbor → Check if neighbors are in Full state.
✅ debug ip ospf adj → Identify adjacency issues.

🔴 If neighbors are stuck in INIT or EXSTART:

• Check MTU mismatch → show ip ospf interface
• Check Hello/Dead timer mismatch → show ip ospf interface
• Check Network Type mismatch (Broadcast, P2P, etc.)
• Check Area ID mismatch → show ip ospf interface brief
• Check for authentication issues → show running-config | section ospf

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Step 3: Validate OSPF Route Advertisements

✅ show ip route ospf → Confirm OSPF routes are present.
✅ show ip ospf database → Verify LSAs are exchanged properly.
✅ debug ip ospf lsa → Check if LSAs are being received.

🔴 If missing routes:

• Check if the interface is passive → show ip ospf interface brief
• Check LSA filtering or stub area restrictions
• Check for route summarization issues

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Step 4: Check OSPF Path Selection & Costs

✅ show ip ospf interface → Verify OSPF cost settings.
✅ show ip ospf database router → Inspect LSA details.
✅ show ip route <destination> → Check the chosen OSPF path.

🔴 If unexpected routes appear:

• OSPF cost may be too high/low → Adjust with ip ospf cost <value>
• Check for ECMP issues (Equal-Cost Multipath Routing)

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Step 5: Verify External Route Redistribution (If applicable)

✅ show ip ospf database external → Check Type-5 LSAs.
✅ show running-config | section redistribute → Ensure correct redistribution.
✅ debug ip ospf events → Monitor redistribution behavior.

🔴 If external routes are missing:

• Redistribution misconfigured → redistribute <protocol> subnets
• ACLs or route-maps filtering OSPF routes
• NSSA blocking external routes

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Step 6: Debug & Monitor Logs

✅ debug ip ospf hello → Analyze Hello packet issues.
✅ debug ip ospf packet → Monitor overall OSPF packet exchange.
✅ show logging → Check for error messages.

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