OSPF(Open Shortest Path First) Overview
OSPF(Open Shortest Path First) is a Link-State Routing Protocol. It is also Classless and an Open, Standard Protocol used by all vendors. OSPF uses Dijsktra Algortihm, a Shortest Path First Algorithm(SPF) to determine the best path to a destination network.
OSPF provide neighbourship between other OSPF routers by the help of OSPF messages. With these messages, different tables are built. And OSPF routing mechanims works with this tables. Basically, there are three tables in OSPF. These tables are :
• Neighboring Table
• Topology Table
• Routing Table
In OSPF, all routers build its own Topology Table and has the full view of the network. It calculates the next hope independently from other routers. This is also the characteristic of Link-State Protocol. As you remember, Link-State protocols are “OSPF” and “IS-IS”. In Distance-Vector protocols, the situation is not like this. Distance-Vector protocols needs a neighbor to know about the network.
Hierarchical OSPF and Area Types
OSPF has an hierarchical architecture. It uses “Areas” for hieararcy. There is a Backbone Area as Area 0 in the center for all OSPF Networks. Around this Backbone Area, Area 0, there are some other Areas. All these areas are connected to this Area 0. If there is a uncontinuous structure, Virtual-Links can be used to connect an area to the Backbone Area(Area 0).
With this hierachical architecture of OSPF, you can divide your network into different small areas and by doing this, you can reduce overhead of this small areas. You can also use specific areas in OSPF network. These areas will be explain detailly later.
Here, let’s only give the area types used in OSPF: