OSPF is a link-state routing protocol and it’s one of the routing protocols you need to understand if you want to do the Cisco CCNA, CCNP or CCIE R&S exam(s). In this lesson I’ll explain the basics of OSPF to you and you will learn how and why it works.
Link-state routing protocols are like your navigation system, they have a complete map of the network. If you have a full map of the network you can just calculate the shortest path to all the different destinations out there. This is cool because if you know about all the different paths it’s impossible to get a loop since you know everything! The downside is that this is more CPU intensive than a distance vector routing protocol. It’s just like your navigation system…if you calculate a route from New York to Los Angeles it’s going to take a bit longer than when you calculate a route from one street to another street in the same city.
An online poll done by a highly respectable network media website showed that the average OSPF network contains 50-75 areas. Keep in mind that not every single building has to be its own OSPF area, a general rule of thumb when designing OSPF area’s is that a single area could contain up to 250 routers and a few hundred intra-area routes.
To configure a new area the command is identical to configuring the backbone area but instead of specifying area 0 after the network statement you specify the new area number. Remember Area 0 is the backbone area and all traffic traversing the network from one area to another area MUST!!! traverse the backbone area.
An OSPF network can be divided into sub-domains called areas. An area is a logical collection of OSPF networks, routers, and links that have the same area identification. A router within an area must maintain a topological database for the area to which it belongs. The router doesn’t have detailed information about network topology outside of its area, thereby reducing the size of its database.