Every router that enable ospf routing protocol will try to achieve adjacency condition with neighbor routers. The following as the example topology :
Imagine that all of the routers are do adjacency each others, it’s looks like that :
With 6 Routers in one topology with each router send the ospf packet it will cause the issues. every routers will do the full adjacency with the neighboor routers. Those process will make hight consume for the resource of CPU and RAM . and it will consumes hight bandwidth for the sending of ospf packet.
For the solved that, on OSPF process there are DR (Designated Router) and BDR (Backup Designated Router) election process, so other routers will only send adjacency with DR and BDR as the following topology :
With DR and BDR concept routers do not require adjacency with all routers, but only need to adjacency with DR and BDR.
DR (Designated Router) is a primary router that controlled the ospf running proccess. If on one situation the DR is down, then BDR will become DR to continue ospf proccess keep alive.
For DR and BDR Selection are based on interface priority on each routers. Router with 1st highest priority will become DR and router with 2nd highest priority will become BDR. But if all routers in topology has the same priority, it will select by highest router-id. The explanation of router id is already discuss on previous article https://www.taufiknurhuda.web.id/2024/08/28/ospf-router-id-concept/
The following example that DR & BDR selection based on Priority. R2 has 1st highest priority will become DR and R1 has 2nd highest priority will become BDR.
Second example that if all of the routers have same priority it will specify based on highest router id. R4 has 1st highest router id will become DR and R3 has 2nd highest router id will become BDR.
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