OSPF Routing

OSPF is a link-state routing protocol commonly used in enterprise networks.

Key Concepts

• * Areas: Hierarchical design with Area 0 as backbone
• * Router ID: Unique identifier (usually loopback IP)
• * Cost: Metric based on interface bandwidth
• * LSA: Link State Advertisements for topology sharing
• * DR/BDR: Designated routers on multi-access networks

Cisco IOS Example

! Enable OSPF process
router ospf 1
 router-id 10.0.0.1

! Advertise networks into Area 0
 network 10.10.10.0 0.0.0.255 area 0
 network 10.10.20.0 0.0.0.255 area 0

! Passive interface (no OSPF hellos)
 passive-interface GigabitEthernet0/1

! Default route redistribution
 default-information originate

Verification Commands

show ip ospf neighbor
show ip ospf interface brief
show ip route ospf
show ip ospf database

• * Use loopbacks for Router ID: Ensures stability
• * Set reference bandwidth: auto-cost reference-bandwidth 10000 for 10G networks
• * Use passive-interface default: Only enable OSPF where needed
• * Summarize at area boundaries: Reduce LSA flooding
• * Use authentication: MD5 or SHA for security
• * Document area design: Keep topology diagrams current

• Neighbor stuck in INIT/2-WAY:

  Check MTU mismatch, hello/dead timers, area ID

• Routes not appearing:

  Verify network statements, check for filtering

• Suboptimal routing:

  Review cost settings, check for summarization issues