OSPF Design for Enterprise Networks on Cisco IOS

Introduction

OSPF on Cisco IOS is the most common IGP (Interior Gateway Protocol) in enterprise networks. Unlike the MikroTik OSPF guide, Cisco's OSPF configuration uses a different syntax and has unique enterprise features like route redistribution, virtual links, and stub areas.

OSPF Fundamentals

OSPF is a link-state protocol — every router knows the complete topology (LSDB). Routers calculate shortest paths using Dijkstra's algorithm.

Key Terms:

Router ID: Unique identifier for each OSPF router (highest loopback IP by default)
Area: Group of routers sharing the same LSDB
ABR: Area Border Router — connects multiple areas
ASBR: Autonomous System Boundary Router — redistributes external routes into OSPF
LSA: Link State Advertisement — the unit of OSPF topology information

Basic OSPF Configuration

CISCO

! Enable OSPF process (number is local only, doesn't need to match peers)
router ospf 1

! Set Router ID explicitly (best practice)
router-id 10.0.0.1

! Advertise networks in OSPF
network 10.0.0.0 0.0.0.255 area 0     ! All /24 in 10.0.0.x → backbone area
network 192.168.1.0 0.0.0.255 area 1  ! Branch network → area 1
network 10.10.10.10 0.0.0.0 area 0    ! Loopback → backbone

! Or configure OSPF per-interface (modern recommended approach)
interface GigabitEthernet0/0
 ip ospf 1 area 0
 ip ospf cost 10
 ip ospf hello-interval 5
 ip ospf dead-interval 20

OSPF Authentication (MD5)

Always enable authentication to prevent rogue router injection:

CISCO

! Interface-level MD5 auth
interface GigabitEthernet0/0
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 YourSecretKey

! Area-level auth (applies to all interfaces in area)
router ospf 1
 area 0 authentication message-digest

Area Types and Design

CISCO

! Regular area (full LSA flooding)
router ospf 1
 area 1 range 192.168.1.0 255.255.255.0  ! Summarize area 1 at ABR

! Stub area (no external LSAs from ASBR)
router ospf 1
 area 2 stub
! All routers in area 2 must also have: area 2 stub

! Totally Stubby area (no external, no summary LSAs — only default route)
router ospf 1
 area 2 stub no-summary  ! Only on ABR
! Regular routers in area: area 2 stub

! NSSA (Not-So-Stubby Area) — allows ASBR inside stub area
router ospf 1
 area 3 nssa

Route Redistribution

Import routes from other protocols into OSPF:

CISCO

! Redistribute connected networks
router ospf 1
 redistribute connected subnets metric 10 metric-type 1

! Redistribute static routes
router ospf 1
 redistribute static subnets

! Redistribute from BGP
router ospf 1
 redistribute bgp 65001 subnets metric 10

! Redistribute from EIGRP with filtering
router ospf 1
 redistribute eigrp 10 subnets route-map EIGRP-TO-OSPF

route-map EIGRP-TO-OSPF permit 10
 match ip address prefix-list ALLOWED-PREFIXES
ip prefix-list ALLOWED-PREFIXES permit 10.0.0.0/8 le 24

BFD for Sub-Second Failure Detection

CISCO

! BFD replaces OSPF dead-interval for faster failure detection
interface GigabitEthernet0/0
 bfd interval 300 min_rx 300 multiplier 3  ! Detect failure in 900ms

router ospf 1
 bfd all-interfaces  ! Enable BFD for all OSPF interfaces

Verifying OSPF

CISCO

! View OSPF neighbors
show ip ospf neighbor

! Detailed neighbor info
show ip ospf neighbor detail

! View OSPF link-state database
show ip ospf database

! View specific LSA type
show ip ospf database router      ! Type 1 LSAs
show ip ospf database network     ! Type 2 LSAs
show ip ospf database summary     ! Type 3 LSAs (inter-area)
show ip ospf database external    ! Type 5 LSAs (external routes)

! View OSPF routes
show ip route ospf

! View OSPF interface info
show ip ospf interface brief

! Check process info
show ip ospf

Common OSPF Issues

CISCO

! Issue: Neighbors stuck in EXSTART/EXCHANGE
! Cause: MTU mismatch
interface GigabitEthernet0/0
 ip ospf mtu-ignore

! Issue: Neighbors not forming (check timers)
! Hello/Dead must match between neighbors
show ip ospf interface GigabitEthernet0/0 | include intervals

! Issue: Route not appearing in table
! Check that network statement covers the interface
debug ip ospf events
debug ip ospf adj

! Issue: Flapping adjacency
! Look for BFD mismatches or unreliable links
show ip ospf statistics