OSPF DR-BDR Election

If a router running OSPF has to exchange routes with another router running OSPF, then a nighborship between the 2 routers is not sufficient. The 2 routers have to form am adjacency.
2 routers forming an adjacency depends on the type of network used.

In case of a BMA or an NBMA network, a DR-BDR election takes place to elect a DR(Designated Router) and a BDR(Backup Designated Router).
All other routers in the network(DROthers) will form full adjacencies with the DR and the BDR.

The topology is an example for a BMA network. Routers R1 through R5 will send and receive Hello Packets and they will form neighborship with each other. Since this is a BMA network, all these routers will not form adjacencies with each other. Routers R1 through R5 will participate in an election to elect the DR and BDR.

The Router with the highest priority will become the DR and the router with the second highest priority will become the BDR. The OSPF priority for all the routers are the same by default. The value is 1. That means R1 through R5 have the same router priority. In this case, the router ID breaks the tie. The router with the highest router ID will become the DR and the router with the second highest router ID will become the BDR. 

In the example, considering the priorities to be the default on all the routers, we can come to a conclusion that R5 is the DR and R3 is the BDR. 

R1, R2 and R4 will form neighborship with each other and also the DR and BDR but they will form adjacencies only with DR and BDR (R5 and R3). R1, R2 and R4 will exchange their LSDBDs with R5 and R3. R1, R2 and R4 will use the multicast address 224.0.0.6 to send routing updates to DR or BDR. In turn DR and BDR sends these routing updates to all other routers (DROthers) on the multicast address 224.0.0.5.

If the DR goes down, the BDR takes its place. Doing this in a BMA network reduces a lot of network traffic as the DR and BDR are considered to be the central point for update traffic.

A similar thing happens with an NBMA network also.