Symptoms

• Neighbors appear in "show ip ospf neighbor" but state is Init
• No route exchange occurring
• Adjacency never progresses to Full state

Common Causes

• Unicast connectivity issues (Layer 2/3 problems)
• Access control lists blocking OSPF traffic
• Firewall blocking multicast 224.0.0.5/224.0.0.6
• MTU mismatches between neighbors

Troubleshooting Steps

# Check basic connectivity
ping 192.168.1.2

# Verify OSPF interface configuration
show ip ospf interface ethernet0/0

# Check for MTU issues
show interface ethernet0/0 | include MTU

# Verify multicast is working
ping 224.0.0.5

# Check ACLs
show access-lists
show ip interface ethernet0/0

Symptoms

• Neighbors reach Exstart but never progress to Exchange
• Database Description (DD) packets being exchanged repeatedly
• Master/Slave relationship not established

Common Causes

• MTU mismatches (most common)
• Duplicate Router IDs
• Unidirectional link failure
• High packet loss or delay

Resolution

# Check MTU settings
show ip ospf interface ethernet0/0
show interface ethernet0/0

# Configure MTU ignore (temporary fix)
interface ethernet0/0
ip ospf mtu-ignore

# Check for duplicate Router IDs
show ip ospf
show ip ospf database

# Verify bidirectional connectivity
ping 192.168.1.2 source ethernet0/0

Symptoms

• Neighbors not appearing in neighbor table
• Authentication error messages in logs
• Hello packets being dropped

Common Causes

• Mismatched authentication types
• Incorrect passwords/keys
• Wrong key ID numbers
• Time synchronization issues (for cryptographic auth)

Troubleshooting

# Check authentication configuration
show ip ospf interface ethernet0/0

# Verify authentication type matches
show running-config interface ethernet0/0

# Check key configuration
show key chain ospf-keys

# Enable authentication debugging
debug ip ospf adj
debug ip ospf packet

Symptoms

• Expected routes not appearing in routing table
• Intermittent connectivity to destinations
• Routes present in LSDB but not in RIB

Troubleshooting Process

# Check OSPF database for route
show ip ospf database

# Verify route calculation
show ip ospf statistics

# Check routing table
show ip route ospf

# Look for SPF calculation issues
show ip ospf spf-log

# Check for route filtering
show ip prefix-list
show route-map

Symptoms

• Traffic taking longer paths than expected
• Load balancing not working properly
• Backup paths being used instead of primary

Common Causes

• Incorrect interface costs
• Area design problems
• Administrative distance manipulation
• Route redistribution issues

Analysis Commands

# Check interface costs
show ip ospf interface brief

# Verify SPF calculation
show ip ospf database router

# Check route selection
show ip route 192.168.1.0 255.255.255.0 longer-prefixes

# Analyze path selection
traceroute 192.168.1.1

Symptoms

• Long delays before alternative paths are used
• Extended periods of unreachability
• Frequent SPF recalculations

Optimization Techniques

# Tune OSPF timers
interface ethernet0/0
ip ospf hello-interval 5
ip ospf dead-interval 20

# Optimize SPF scheduling
router ospf 1
timers throttle spf 100 500 5000

# Enable fast convergence features
router ospf 1
fast-reroute per-prefix enable

Symptoms

• Routes constantly appearing and disappearing
• High SPF calculation frequency
• Unstable network performance

Common Causes

• Unstable physical links
• Aggressive timer settings
• Hardware issues
• Power or environmental problems

Stabilization Techniques

# Implement route dampening
router ospf 1
area 0 range 192.168.0.0 255.255.0.0 cost 100

# Adjust LSA throttling
router ospf 1
timers throttle lsa all 100 500 5000

# Enable BFD for faster failure detection
interface ethernet0/0
ip ospf bfd

Symptoms

• Inconsistent LSDB across routers
• Routes calculated incorrectly
• LSA age showing maximum value

Detection and Resolution

# Check LSA consistency
show ip ospf database

# Look for corruption indicators
show ip ospf database router self-originate

# Force LSA refresh
clear ip ospf process

# Verify database synchronization
show ip ospf statistics

Symptoms

• Router running out of memory
• OSPF process crashes or restarts
• Inability to store new LSAs

Monitoring and Mitigation

# Check memory usage
show memory summary
show ip ospf database database-summary

# Monitor LSA counts
show ip ospf statistics

# Implement database overflow protection
router ospf 1
area 0 database-filter all out
max-lsa 10000 warning-only