Please explain the concept of OSPF DR BDR Election practically?

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CCNA Routing and Switching

                                                                OSPF DR , BDR Election Process : 

 

DR/BDR Election :  Intially when we enable ospf on routers then they start sending hello messages to each other . Then at The End of Two-Way State DR/BDR Election is done.

Election Process: 

1. Priority : Interface on which ospf is enabled by default priority is 1 but it can range from 0-255 and if priority of a interface is 0 mean it will not participate in DR/BDR election .

-- As on all interface priority is 1 by default  therefore it will be a tie . Then election process will move to router-id.

- Higher Priority will be preffered.

To check Priority on a interface : show ip ospf neighbor

ex. 

R1(config)#do show ip ospf neighbor 

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   2WAY/DROTHER    00:00:32    12.1.1.2        FastEthernet0/0
3.3.3.3           1   FULL/BDR        00:00:33    12.1.1.3        FastEthernet0/0
4.4.4.4           1   FULL/DR         00:00:36    12.1.1.4        FastEthernet0/0

To change priority of a interface : 

interface fa0/0

ip ospf priority ?- 0-255

exit

 

2. Router-Id : As Every router in case of ospf should have unique router-id to become neighbors .

-- So Higher Router-Id will be preffered.

As in our example Router R4 has higher Router-Id 4.4.4.4 So he will be DR and Router R3 having Router-Id 3.3.3.3 so he will be BDR.

to check who is DR/BDR : show ip ospf neighbor

ex. 

R1(config)#do show ip ospf neighbor 

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2           1   2WAY/DROTHER    00:00:32    12.1.1.2        FastEthernet0/0
3.3.3.3           1   FULL/BDR        00:00:33    12.1.1.3        FastEthernet0/0
4.4.4.4           1   FULL/DR         00:00:36    12.1.1.4        FastEthernet0/0

..........................................................................................................................................................................................................

-> Router which are nor DR neither BDR will be called as DROTHER. 

->DROTHER routers will exchange database with only DR and BDR on multicast address 224.0.0.6 and then DR will forward that update to all DROTHER routers.

-> But communication among all routers will be direct.