by: Networking

OSPF is an open standard (non-proprietary) routing protocol. It is a link-state protocol that allows all routers on the network to map the entire topology and make their own forwarding decisions based on that complete picture.

Given the scenario below, PC1 needs to reach PC2:

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Router1

enable 
configure terminal 
no ip domain lookup
line con 0
logging synchronous
hostname Router1
interface g0/0
ip address 192.168.3.1 255.255.255.252
no shutdown
interface g0/1
ip address 192.168.1.1 255.255.255.0
no shutdown

Router2

enable 
configure terminal 
no ip domain lookup
line con 0
logging synchronous
hostname Router2
interface g0/0
ip address 192.168.3.2 255.255.255.252
no shutdown
interface g0/1
ip address 192.168.2.1 255.255.255.0
no shutdown

Example of a Single Area configuration:

Router1

router ospf 1
network 192.168.1.0 0.0.0.255 area 0
network 192.168.3.0 0.0.0.3 area 0
passive-interface GigabitEthernet0/1

Router2

router ospf 1
network 192.168.2.0 0.0.0.255 area 0
network 192.168.3.0 0.0.0.3 area 0
passive-interface GigabitEthernet0/1

For simple networks with few routers this works perfectly. The problem starts when there are many routers, say 50. When one link goes down, all routers begin exchanging link-state messages. This overhead can be reduced by segmenting the network into areas:

Configuring the interfaces according to the diagram above:

RouterA1

enable 
configure terminal 
hostname RA1
no ip domain lookup
line con 0
logging synchronous
interface g0/0
ip address 10.0.0.2 255.255.255.0
no shutdown
interface s0/0/0
ip address 10.0.1.1 255.255.255.0
no shutdown

RouterA3

enable 
configure terminal 
hostname RA3
no ip domain lookup
line con 0
logging synchronous
interface s0/0/1
ip address 10.0.3.1 255.255.255.0
no shutdown
interface g0/1
ip address 10.0.2.2 255.255.255.0
no shutdown

RouterB2

enable 
configure terminal 
hostname RB2
no ip domain lookup
line con 0
logging synchronous
interface g0/0
ip address 10.0.6.1 255.255.255.0
no shutdown
interface g0/1
ip address 10.0.5.2 255.255.255.0
no shutdown

Router Gateway

enable 
configure terminal 
hostname Gateway
no ip domain lookup
line con 0
logging synchronous
interface s0/0/0
ip address 10.0.4.2 255.255.255.0
no shutdown
interface s0/0/1
ip address 10.0.7.2 255.255.255.0
no shutdown
interface s0/1/0
ip address 10.0.1.2 255.255.255.0
no shutdown
interface s0/1/1
ip address 10.0.3.2 255.255.255.0
no shutdown

RouterA2

enable 
configure terminal 
hostname RA2
no ip domain lookup
line con 0
logging synchronous
interface g0/0
ip address 10.0.0.1 255.255.255.0
no shutdown
interface g0/1
ip address 10.0.2.1 255.255.255.0
no shutdown

RouterB1

enable 
configure terminal 
hostname RB1
no ip domain lookup
line con 0
logging synchronous
interface g0/0
ip address 10.0.6.2 255.255.255.0
no shutdown
interface s0/0/1
ip address 10.0.7.1 255.255.255.0
no shutdown

RouterB3

enable 
configure terminal 
hostname RB3
no ip domain lookup
line con 0
logging synchronous
interface s0/0/0
ip address 10.0.4.1 255.255.255.0
no shutdown
interface g0/1
ip address 10.0.5.1 255.255.255.0
no shutdown

Configuring OSPF according to the areas in the diagram:

RouterA1

router ospf 1
network 10.0.0.0 0.0.0.255 area 1
network 10.0.1.0 0.0.0.255 area 1

RouterA2

router ospf 1
network 10.0.0.0 0.0.0.255 area 1
network 10.0.2.0 0.0.0.255 area 1

RouterA3

router ospf 1
network 10.0.2.0 0.0.0.255 area 1
network 10.0.3.0 0.0.0.255 area 1

Router Gateway

router ospf 1
network 10.0.1.0 0.0.0.255 area 1
network 10.0.3.0 0.0.0.255 area 1
network 10.0.7.0 0.0.0.255 area 2
network 10.0.4.0 0.0.0.255 area 2

RouterB1

router ospf 1
network 10.0.7.0 0.0.0.255 area 2
network 10.0.6.0 0.0.0.255 area 2

RouterB2

router ospf 1
network 10.0.6.0 0.0.0.255 area 2
network 10.0.5.0 0.0.0.255 area 2

RouterB3

router ospf 1
network 10.0.5.0 0.0.0.255 area 2
network 10.0.4.0 0.0.0.255 area 2

Note:

  • Routers A* belong to area 1, so they are called Internal routers;
  • Routers B* belong to area 2, so they are called Internal routers;
  • Router Gateway has interfaces in both area 1 and area 2, so it is called an Area Border Router;

The router that connects to outside the network requires this configuration:

router ospf 1
default-information originate
ip route 0.0.0.0 0.0.0.0 g0/0

This tells all other routers which one is the ‘gateway of last resort‘ when a destination network is unknown.

To verify the configuration on each router, use the following commands:

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

Reference Bandwidth (for optimized route calculation):

router ospf 1
auto-cost reference-bandwidth 10000
ip ospf cost 22222

Set all interfaces as passive by default:

passive-interface default
no passive-interface Serial0/0/0
no passive-interface Serial0/0/1

To enable authentication for OSPF, apply the following commands on all interfaces on all routers:

ip ospf message-digest-key 1 md5 PASSWORD
ip ospf authentication message-digest

Remember to replace ‘PASSWORD’ with a secret of your choice.

Learn more about [OSPF Single – CertBros]

Learn more about [OSPF Multi – CertBros]

The previous post was about RIPv2 [Read It].

The following post is about EIGRP [Read It].