Open Shortest Path First Protocol | Link State Protocol

Fig 1: Open Shortest Path First Protocol

Open Shortest Path First is the dynamic routing protocol and this is layer three means network layer protocol. This protocol is the most using protocol in the industry. And this protocol is the protocol that is also known as the link state protocol. The word link means interface of router and the word state is "description of interface and it's connected to neighbor router".

Major Points

1. Open Shortest Path First is the link state protocol.
2. OSPF is the Interior gateway protocol that configure with in the single autonomous system.
3. OSPF Administrative Distance value is 110 in cisco devices and it is the configurable parameter. Basically AD value describe the priority of routing information.
4. OSPF based on the Dijkstra's Algorithm and this algorithm is also known as the shortest path first algorithm and the other third one name is link state algorithm.
5. There are the following three tables that exist 
• Neighbor table  that has the neighbors information 
• Database table that is also known as the topology table 
• Routing Table and in this table only best paths are  exist.
6. Metric/Cost: when cost is less than it will be preferred path according to the Shortest Path Algorithm. Means if the bandwidth is greater than that path will be preferred because OSPF's cost is bandwidth.        
• [Cost ∝ 1/Bandwidth]
7. LSA means link state advertisement and in OSPF LSA is the source of the exchanging information.
8. Area (logical Area) basically it is the collection of routers. So there are the two types of area:
•  Single Area: In this area type only single area exist that consist of the multiples routers.
•  Multi Area: In multi area there are the multiples areas in which area 0 is the backbone area  and the other areas are the non backbone area. All the none backbone area should have to directly connected to the backbone area means with area 0.

Nutt And Bolts

Before this configuring protocol on the devices we should have to know that these three important piece of concept that are very important:

• Process ID : Process ID is the id that is the identification of the OSPF that is 2^16 which is equal to the 65535 and the range we can set is 1-65535. This is the compulsory parameter which is very important.
• Wildcard Mask : OSPF support the wildcard mask that is the inverse of the subnet mask.
• OSPF Area ID  : Area ID is the identification of the area and area 0 is backbone area and area 1-n are the non backbone areas.