# Ubuntu/Debian
curl -s https://deb.frrouting.org/frr/keys.gpg | \
  sudo gpg --dearmor -o /usr/share/keyrings/frr.gpg

echo "deb [signed-by=/usr/share/keyrings/frr.gpg] \
  https://deb.frrouting.org/frr $(lsb_release -s -c) frr-stable" | \
  sudo tee /etc/apt/sources.list.d/frr.list

sudo apt update && sudo apt install -y frr frr-pythontools

# CentOS/Rocky Linux
sudo dnf install -y https://rpm.frrouting.org/repo/frr-stable-repo-1-0.el9.noarch.rpm
sudo dnf install -y frr

# Enable OSPF daemon
sudo sed -i 's/^ospfd=no/ospfd=yes/' /etc/frr/daemons
sudo sed -i 's/^staticd=no/staticd=yes/' /etc/frr/daemons
sudo systemctl enable frr && sudo systemctl restart frr

# Open the FRR configuration shell
sudo vtysh

configure terminal

# Set router ID (use loopback IP for stability)
router ospf

  # Stable router-id (loopback address)
  ospf router-id 10.255.255.1

  # Advertise networks into OSPF
  # Syntax: network <prefix> area <area-id>
  network 10.0.1.0/30 area 0
  network 10.0.2.0/30 area 0
  network 10.255.255.1/32 area 0    # Loopback

  # Enable OSPF logging
  log-adjacency-changes detail

exit

write memory
end

Router1 (10.255.255.1) --- 10.0.1.0/30 --- Router2 (10.255.255.2)
                       \                  /
                        10.0.2.0/30 --- /

sudo vtysh
configure terminal

router ospf

  # Backbone area (area 0) — must be configured on all ABRs
  network 10.0.1.0/30 area 0
  network 10.255.255.1/32 area 0

  # Area 1 — connects to area 0 through this router (ABR role)
  network 192.168.1.0/24 area 1
  network 192.168.2.0/24 area 1

  # Area 2 — another spoke area
  network 172.16.0.0/16 area 2

exit

# Configure virtual link if area 0 connectivity is broken
# (connect area through another area using virtual-link)
router ospf
  area 1 virtual-link 10.255.255.3   # Router ID of the far end

write memory
end

# Verify area assignments
show ip ospf
show ip ospf database

sudo vtysh
configure terminal

# Set reference bandwidth to match your network (default is 100 Mbps)
# For gigabit networks:
router ospf
  auto-cost reference-bandwidth 1000

exit

# Set interface cost manually (overrides auto-cost)
interface eth0
  ip ospf cost 10       # Lower cost = preferred path

interface eth1
  ip ospf cost 100      # Higher cost = backup path

write memory
end

# Verify current costs
show ip ospf interface eth0

sudo vtysh
configure terminal

router ospf
  # Enable ECMP (equal-cost multi-path)
  maximum-paths 4

write memory
end

# Verify routing table shows multiple paths
show ip route ospf

sudo vtysh
configure terminal

# Configure OSPF timers on an interface (faster convergence)
interface eth0
  # Hello interval: 5s, Dead interval: 20s (default: 10/40)
  ip ospf hello-interval 5
  ip ospf dead-interval 20

  # Set interface network type
  # point-to-point: no DR/BDR election (use on P2P links)
  ip ospf network point-to-point

  # broadcast: DR/BDR election (default for Ethernet)
  # ip ospf network broadcast

  # Set DR priority (higher = more likely to become DR)
  # Set to 0 to never become DR
  ip ospf priority 1

exit

# MD5 authentication for OSPF neighbors (prevents rogue routers)
interface eth0
  ip ospf authentication message-digest
  ip ospf message-digest-key 1 md5 MySecretKey

write memory
end

# Verify neighbor state
show ip ospf neighbor
show ip ospf neighbor detail

sudo vtysh
configure terminal

router ospf

  # Redistribute connected interfaces (non-OSPF networks)
  redistribute connected

  # Redistribute static routes
  redistribute static metric 20 metric-type 2

  # Redistribute BGP routes into OSPF
  redistribute bgp metric-type 1

  # Redistribute with a route map (filter which routes enter OSPF)
  redistribute connected route-map CONN-TO-OSPF

exit

# Route map to filter redistributed routes
ip prefix-list ALLOWED-NETWORKS seq 10 permit 192.168.0.0/16 le 24

route-map CONN-TO-OSPF permit 10
  match ip address prefix-list ALLOWED-NETWORKS
  set metric 50

write memory
end

# Check redistributed routes in OSPF database
show ip ospf database external

sudo vtysh
configure terminal

router ospf

  # Configure area 1 as a stub (no external LSAs; gets default route instead)
  area 1 stub

  # Configure area 2 as NSSA (can import external routes locally)
  area 2 nssa

  # Totally stubby area (no external OR inter-area LSAs)
  area 1 stub no-summary

exit

write memory
end

# On the ABR, verify stub configuration
show ip ospf
# Look for: "Area 1 is a stub area"

# Check that hello timers match on both sides
show ip ospf interface eth0
# "Hello due in X sec, Dead interval X sec" — must match peer

# Verify MTU matches (mismatched MTU blocks adjacency)
ip link show eth0 | grep mtu
# If MTU differs, configure on the interface:
sudo vtysh -c "conf t" -c "interface eth0" -c "ip ospf mtu-ignore"

# Verify OSPF is running
show ip ospf

# Check LSA database
show ip ospf database
show ip ospf database router

# Verify network statement covers the interface
show ip ospf interface

# Check for route filtering
show ip route ospf

# Check FRR logs
sudo journalctl -u frr -n 100 | grep -i "ospf\|adj"

# Monitor neighbor changes in real time
sudo vtysh
debug ospf adj
debug ospf hello
# Watch output for "neighbor state change"
no debug ospf all   # Disable when done

# Verify routes exist in the routing table before redistribution
show ip route connected
show ip route static

# Check OSPF external LSAs
show ip ospf database external