by: Networking1 Comment on OSPF on Cisco Routers

OSPF is one open standard (no proprietary) routing protocol. It is a link-state protocol and allows all the routers on the network to map the whole network to make its own decisions based on the big print.

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 it works perfectly. The problem start when there are many routers, let’s say 50. When one link goes down all the routers start to change list state messages and this could be avoided be segmenting the network in zones:

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 the OSPF according to the Areas on 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* are area 1, so they are colled Internal;
  • Routers B* are area 2, so they are colled Internal;
  • Router Gateway has interfaces in area 1 and area 2, so it is called Area Border or Backbone;

The router gateway to outside the network has to have this config:

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

This will inform all the other routers who of them is the ‘gateway of last resort‘ when one network is unknown.

To check the configuration in each Router issue the 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 apply authentication to join the OSPF, issue the same command in all interfaces in all routers:

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

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

Learn more about [OSPF Single – CertBros]

Learn more about [OSPF Multi – CertBros]

The preview Post was about RIPv2 [Read It].

The following Post is about EIGRP [Read It].

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