OSPF DR and BDR election

OSPF elect a Designated Router (DR) for each multi- access networks, accessed via multicast address 224.0.0.6. For redundancy purposes, a Backup Designated Router (BDR) is also elected. 

DR and BDR election

• The router with the highest priority becomes the DR; second highest becomes the BDR. If there is a tie in priority, Whichever router has the highest Router ID will become the DR. 
• By default router priority will be same.We can change it if we need it
• Default priority on Cisco routers is 1.If we set Router priority is O, that router will not participate in DR/BDR election
• In FrameRelay (NBMA -non broadcast multi access) network ,HUB Must be elected as DR .We can do this by changing router priority

To change the priority on an interface: 

  Router(config)#Interface  fa 0/0
  Router(config-if)# ip ospf priority 125 

OSPF routers will form adjacencies with the DR and BDR. If a change occurs to a link, the update is forwarded only to the DR, which then  forwards it to all other routers. This greatly reduces the flooding of LSAs. 

Note: The DR election process is not preemptive .Thus, if a router with a higher priority is added to the network, it will not automatically supplant an existing DR. We need to clear OSPF process for another DR/BDR election,which is not good in a production enviroment 

Related articles

• Basic Notes on OSPF (Open Shortest Path First)
• OSPF neighbors
• How to Prepare for NETWORK ENGINEER INTERVIEW QUESTIONS
• VLAN explained with Interview Questions

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