Jorge Martinez de Salinas

In a previous post we talked about RIP. Today I’m going to show you how to configure OSPF. OSPF is a dynamic link-state routing protocol used in IP networks. OSPF is perhaps the most widely used interior gateway protocol (IGP) in large enterprise networks. OSPF exceeds RIP in many aspects:

• It has very low convergence times.
• When no topology changes occur, OSPF is very quiet.
• OSPF enables network subdivision into areas.
• Supports authentication.
• Uses multicast.
• Only routing changes are propagated, not the full routing table like in the RIP case.
• …

To configure OSPF in your cisco router you should follow these steps:

fry> enable 
fry# configure terminal 
fry(config)#router ospf 1 
fry(config-router)#network 192.168.2.0 0.0.0.255 area 0 
fry(config-router)#network 192.168.3.0 0.0.0.255 area 0
fry(config-router)#network 192.168.4.0 0.0.0.255 area 0

You have to specify all the interfaces in which you want to run OSPF with network commands. 0.0.0.255 is a wildcard and means that OSPF will run in any interface with an IP address belonging to the 192.168.2.0/24 network. You can also indicate a specific IP address. If the interface’s IP address changes (e.g. from 192.168.4.1 to 192.168.4.2), OSPF will stop running. The configuration is as follows:

fry(config-router)#network 192.168.4.1 0.0.0.0 area 0

To propagate a default route you can execute this command:

fry(config-router)#default-information originate always

One OSPF drawback could be the configuration complexity. However, if the network topology is point-to-point and we only have one area (like in this case) configuration is pretty straightforward.