Cisco OSPF Configuration Guide

OSPF (Open Shortest Path First) is the most widely deployed interior gateway protocol in enterprise networks. This guide covers configuration from basic single-area setups to multi-area deployments.

Basic OSPF configuration

Minimum configuration for a single-area OSPF deployment:

router ospf 1
 router-id 1.1.1.1
 network 192.168.1.0 0.0.0.255 area 0
 network 10.0.0.0 0.0.0.3 area 0

Interface-level OSPF (modern approach)

Preferred on IOS 12.4+ — attach OSPF directly to interfaces:

interface GigabitEthernet0/0
 ip ospf 1 area 0
 ip ospf cost 10

Multi-area OSPF

Divide large networks into areas to limit LSA flooding. The backbone router (area 0 only):

router ospf 1
 router-id 1.1.1.1
 network 10.0.0.0 0.0.0.3 area 0

The ABR (Area Border Router) connecting backbone to area 1:

router ospf 1
 router-id 2.2.2.2
 network 10.0.0.0 0.0.0.3 area 0
 network 192.168.10.0 0.0.0.255 area 1
 network 192.168.20.0 0.0.0.255 area 1

Area 0 (backbone) must connect all other areas. ABRs maintain separate LSDBs per area and summarize routes between them.

Passive interfaces

Prevent OSPF hellos on user-facing interfaces while still advertising the network.

Explicit approach — mark individual interfaces passive:

router ospf 1
 passive-interface GigabitEthernet0/1
 passive-interface GigabitEthernet0/2

Default approach — passive on all, then opt-in routed interfaces:

router ospf 1
 passive-interface default
 no passive-interface GigabitEthernet0/0
 no passive-interface GigabitEthernet0/1

OSPF authentication

MD5 authentication per interface:

interface GigabitEthernet0/0
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 SecretKey123

Redistribution into OSPF

Redistribute static routes and connected interfaces:

router ospf 1
 redistribute static subnets metric 20 metric-type 2
 redistribute connected subnets

Verification commands

Command	Purpose
show ip ospf neighbor	Verify adjacencies
show ip ospf database	View LSDB
show ip route ospf	OSPF routes in routing table
show ip ospf interface	Interface OSPF parameters
debug ip ospf adj	Adjacency formation debug

Common OSPF issues

Neighbors stuck in EXSTART/EXCHANGE: MTU mismatch. Fix with ip ospf mtu-ignore on the affected interface.

Neighbors not forming: Check that area IDs match, hello/dead timers match, and authentication matches if configured.

Route not appearing: Verify the network statement wildcard mask covers the interface IP.

Use our Cisco script generator to generate OSPF configurations automatically.