OSPF Routing Lab

OSPF Overview

OSPF (Open Shortest Path First) is a link-state routing protocol standardised as RFC 2328. Each router builds a complete map of the network topology by exchanging LSAs (Link State Advertisements), then runs Dijkstra's SPF algorithm to find the shortest path to every destination.

Step 1 — Lab Topology

        .1          .2
R1 ----[eth0/1]---[eth0/1]---- R2
10.0.12.0/30              10.0.23.0/30
                       .1          .2
              R2 ----[eth0/2]---[eth0/2]---- R3

Loopbacks:
R1: 1.1.1.1/32
R2: 2.2.2.2/32
R3: 3.3.3.3/32

LANs:
R1: 192.168.1.0/24
R3: 192.168.3.0/24

Step 2 — Interface Configuration

! R1
interface Loopback0
 ip address 1.1.1.1 255.255.255.255
interface GigabitEthernet0/1
 ip address 10.0.12.1 255.255.255.252
 no shutdown
interface GigabitEthernet0/0
 ip address 192.168.1.1 255.255.255.0
 no shutdown

! R2
interface Loopback0
 ip address 2.2.2.2 255.255.255.255
interface GigabitEthernet0/1
 ip address 10.0.12.2 255.255.255.252
 no shutdown
interface GigabitEthernet0/2
 ip address 10.0.23.1 255.255.255.252
 no shutdown

! R3
interface Loopback0
 ip address 3.3.3.3 255.255.255.255
interface GigabitEthernet0/2
 ip address 10.0.23.2 255.255.255.252
 no shutdown
interface GigabitEthernet0/0
 ip address 192.168.3.1 255.255.255.0
 no shutdown

Step 3 — Enable OSPF

! R1 — Single area OSPF
router ospf 1
 router-id 1.1.1.1
 network 10.0.12.0 0.0.0.3 area 0
 network 192.168.1.0 0.0.0.255 area 0
 network 1.1.1.1 0.0.0.0 area 0

! R2
router ospf 1
 router-id 2.2.2.2
 network 10.0.12.0 0.0.0.3 area 0
 network 10.0.23.0 0.0.0.3 area 0
 network 2.2.2.2 0.0.0.0 area 0

! R3
router ospf 1
 router-id 3.3.3.3
 network 10.0.23.0 0.0.0.3 area 0
 network 192.168.3.0 0.0.0.255 area 0
 network 3.3.3.3 0.0.0.0 area 0

Step 4 — Router ID

# Router ID selection priority:
# 1. Manually configured router-id (recommended)
# 2. Highest loopback interface IP
# 3. Highest active physical interface IP

# Always set manually — prevents unexpected changes
router ospf 1
 router-id 1.1.1.1

# After changing router-id, clear OSPF process
clear ip ospf process   # (confirm yes)

Step 5 — Verify Neighbours

# Show OSPF neighbours
show ip ospf neighbor

# Expected output on R2:
Neighbor ID  Pri  State    Dead Time  Address     Interface
1.1.1.1       1  FULL/DR  00:00:38  10.0.12.1   Gi0/1
3.3.3.3       1  FULL/DR  00:00:36  10.0.23.2   Gi0/2

# Neighbour states — must reach FULL for adjacency
# DOWN → INIT → 2-WAY → EXSTART → EXCHANGE → LOADING → FULL

# Show routing table (O = OSPF routes)
show ip route ospf

# Show OSPF database
show ip ospf database

Step 6 — DR/BDR Election

# DR/BDR elected on multi-access networks (Ethernet)
# Election based on: highest priority → highest router-id
# Priority 0 = never become DR/BDR

# Set OSPF priority on interface (higher = more likely DR)
interface GigabitEthernet0/1
 ip ospf priority 255   # guaranteed DR

interface GigabitEthernet0/2
 ip ospf priority 0     # never DR

# Verify DR/BDR
show ip ospf interface GigabitEthernet0/1

Step 7 — OSPF Cost

# Default cost = 100,000,000 / interface bandwidth (bps)
# FastEthernet (100M) = cost 1
# GigabitEthernet (1G) = cost 1 (same as FE — problem!)
# Serial T1 (1.544M) = cost 64

# Fix: change reference bandwidth to 1Gbps
router ospf 1
 auto-cost reference-bandwidth 1000

# Or set cost manually on interface
interface GigabitEthernet0/1
 ip ospf cost 10

# Lower cost = preferred path
show ip ospf interface brief

Step 8 — Multi-Area OSPF

# R2 becomes ABR (Area Border Router) connecting area 0 and area 1
router ospf 1
 router-id 2.2.2.2
 network 10.0.12.0 0.0.0.3 area 0      # backbone
 network 10.0.23.0 0.0.0.3 area 1      # area 1

# R1 in area 0, R3 in area 1
# R3 config:
router ospf 1
 router-id 3.3.3.3
 network 10.0.23.0 0.0.0.3 area 1
 network 192.168.3.0 0.0.0.255 area 1

Step 9 — Passive Interfaces

# Stop sending OSPF Hellos on LAN interfaces (no routers connected)
# Still advertises the network — just doesn't send Hellos
router ospf 1
 passive-interface GigabitEthernet0/0   # LAN interface

# Or set all interfaces passive then enable only on WAN links
router ospf 1
 passive-interface default
 no passive-interface GigabitEthernet0/1   # enable OSPF here

Step 10 — Troubleshooting

# OSPF neighbours won't form? Check:
# 1. Same area number on both sides?
# 2. Same Hello/Dead timers? (must match)
# 3. Same subnet? (must be same network)
# 4. Authentication mismatch?
# 5. MTU mismatch? (causes EXSTART/EXCHANGE loop)

# Check timers
show ip ospf interface GigabitEthernet0/1

# Check MTU
show interface GigabitEthernet0/1 | include MTU

# Debug OSPF (use carefully — can be verbose)
debug ip ospf events
debug ip ospf adj
undebug all   # stop debugging

# Ping across OSPF network
ping 192.168.3.1 source 192.168.1.1

📋 OSPF Command Reference