OSPF Routing

OSPF configuration and best practices for dynamic routing.

OSPFRoutingNetworkingCiscoLink-State

OSPF Overview

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

Basic Configuration

Cisco IOS Example

text
! 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

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

Best Practices

  • 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

Common Issues

  • 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
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