Election of DR

Hello packets have information of DR and BDR
        1. higher priority router will be elected as DR
        2. higher router ID router will be elected as DR

DR and BDR election is used to reduce the flooding of databases with each other on multi-access segments because exchanging database directly will increase database flooding.
DR and BDR also reduce the number of adjacencies on the multi-access segment. Routers will only form an adjacency with DR and BDR.
All DRother(with are not DR and BDR) will remain in 2-way state with each other.
Before DR and BDR election state of the interface is WAIT.

DR & BDR Use Case

On multiaccess segments like Ethernet, OSPF optimizes the LSDB synchronization and the exchange of LSAs.
        * Exactly after two-way state, routers elect DR and BDR before proceeding to next state.
        * Routers establish the exstart state only with DR, and BDR routers on the segment.
        * LSDB is synchronized only between routers and the DR.
        * Routers send multicast LSUs to 224.0.0.6 (DR/BDR).
        * The DR distributes LSUs to other routers on the segment to 224.0.0.5.

When routers form a neighbor relationship on a multiaccess segment, DR, and BDR election takes place when routers are in the two-way state.
        * The router with the highest OSPF priority, or highest router ID if there is a tie, is elected as the BDR First.
        * BDR verify is there any DR present on that segemnt or not
        * If present it will remains status as BDR
        * If DR do not present on segment, BDR promote itself as a DR
        * And then BDR election happens again
        * And the router with the second highest priority or router ID becomes the BDR.

While the DR and BDR establish a neighbor relationship with all routers on the segment, other routers establish full adjacency only with the DR, and BDR. The neighbor state of the other neighbors stays in the two-way state.

Non-DR routers exchange their databases only with the DR. The DR synchronizes any new or changed LSAs with the rest of the routers on the segment.

Here the process explaned one by one:

        1. Router R3 detect a network change
        2. Then Router R3 will send the updates to DR using multicast address 224.0.0.6
        3. Switch gets the multicast update and broadcast to all the routers present on the segment
                * Finally Router R1 as DR, receives the update and install it in database
                * BDR also listen the address 224.0.0.6, so it will also install the update

        4. DR routers flood the update again on multicast address 224.0.0.5 to send the update to other routers present on the segment
        5. Switch flood the updates to all the neighbors available on the segment BDR router ignore this update, because it is already having a database with same seq number, received in earlier update packet For R3 this update will act as ACK from DR
        6. Router R4, DRother router will send a unicast ACK to DR
        7. And this is how a update flood on a multiaccess segment using DR & BDR concept

Master and Slave election

        * Only for the first time router sends an empty DBD with Init flag 1 along with sequence number and MS bit 1.
        * The router with a higher router ID will be the master and the lower one will be the slave
        * To provide reliability to DBD’s master-slave election happens.
Diff in Serial & Ethernet link
        Serial link
                        No DR&BDR election
        Ethernet link
                        DR&BDR election
        ANF(active neighbor field)
                * if both routers dont know about each other then they use ANF 0.0.0.0
                * if both routers know about each other then they use RID of neighbor