OSPF DR and BDR election

OSPF elect a Designated Router (DR) for each multi- access networks, accessed via multicast address 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 
Enhanced by Zemanta

Did You Enjoy this Article ?

If yes, Then enter your email below to get

more articles on CCNA and CCNP in your inbox

For FREE !