OSPF Introduction

In this tutorial I’ll discuss about OSPF (Open Shortest Path First). What is this ? And what is it for ? 😀

OSPF is a dynamic routing protocol capable of maintaining, managing and distribute routing information between networks although that networks can be change dynamically. OSPF including type of IGP (Interior Gateway Protocol) that has ability for Link-State and using Dijkstra Algoritm that more efficient than other IGP. using it’s own routing protocol is protocol 89. When you activate the routing protocol ospf on the router, the router will try to reach adjacency state with neighbors router. the adjacency state is a state where between routers already connected and agreed to running ospf protocol. If the condition is not reachable, then the routers will not be able to do exchage of ospf routing protocol information. and each router only need to reach adjacency state with the router neigbors as the shown bellow.

For reach adjacency state each router will send Hello Package to neighbors router and the other router also send hello package. and if each router receive for hello package then adjacency state will be reach like this.

Hello package delivery using multicast IP 224.0.0.5 . it’s more effective than using broadcast ip so that the ospf hello package only send to router that running ospf routing protocol. multicast ip 224.0.0.5 is using for communication between router ospf. But the router’s DR and BDR are communicating with multicast ip 224.0.0.6 . when the router’s ospf are communicating with DR/BDR router, that’s using multicast ip 224.0.0.5 because multicast ip 224.0.0.6 can only using for the router’s DR and BDR.

OSPF RouterID

Ospf routerid is a unique identity, Every router running ospf must have different routerid and routerid using 32bit format. that’s routerid are using for router identification between one router with another router and it’s using for DR and BRD router selection. Here’s proccess of routerid selection :