On This Article I’ll discuss about concept or how to ospf work. When we enable the OSPF on routers, then the router will be achieve adjacency condition with neighbor router.
The Adjacency condition is a condition where between two routers neigbor already connected and agreed to run ospf routing protocol. If these condition are not achieve, then the routers can’t running the OSPF routing protocol.
On the OSPF adjacency process, only two routers directy connected will be do the adjacency as the following example :
As you can see on the topology above, R1 is only need to adjacency to R2. also R3 is only need to adjacency with R2. on R2 because it connected with both R1 and R3 then the R2 will be adjacency with R1 and R3.
To Achieve adjacency condition, each router will send the “Hello Packet” to neighbor router. When router received the hello packet from neighbor router, then adjacency condition will be achieve. But if router is not achieve hello packet from neighbor router the adjacency condition will not achieve.
As the following bellow, when both routers are send and receive Hello Packet, then adjacency condition will be achieve.
And the following example when only one router send the hello packet to the neighbor but the router self is not receive Hello packet from neighbor, then the adjacency condition will not achieve.
On OSPF Process, the sending of Hello Packet is use multicast IP (224.0.0.5) as the destination. It’s will most efectively because the sending of hello packet will send to only routers ospf-enabled.
As the following above, the ospf hello packet will send only to routers that running ospf using multicast ip 224.0.0.5. Once adjacency achieve, the routes will select the DR (Designated Router) and BDR (Backup Designated Router) where is the DR and BDR as the responsible routers that running ospf. Communication between DR and BDR are use the multicast ip 224.0.0.6.
As the explanation above, The routers will communication each other with multicast ip 224.0.0.5, between non DR/BDR to DR/BDR router. and for multicast ip 224.0.0.6 is used only to commucation between DR and BDR router.