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Facts about DR and BDR selection in OSPF

April 08, 2018 ,

 One of the most asked interview question related to OSPF protocol is how DR and BDR selection procedure takes place in OSPF and people are confused on that some times. We are going to add some facts and details about DR and BDR selection facts in OSPF and is useful for interview.

OSPF: Open Shortest Path First (OSPF) is the most widely used routing protocol in large enterprise networks. OSPF is based on link-state technology by using shortest path first(SPF) algorithm which calculates the shortest path among all the possible path. 

OSPF puts the possible route in the topology map and select the best route to travel which will be the shortest path among all the possible route.

OSPF Broadcast Network
OSPF by default uses Broadcast network type on all types of Ethernet interfaces. In broadcast multi-access networks, we are faced by two challenges in an OSPF environment and they are Multiple adjacencies and Flooding of LSAs. In OSPF, these challenges are solved by electing the DR (designated Router) and the BDR (Backup Designated Router)

Note: DR and BDR selection are required (loop Prevention Mechanism)

Fig 1.1- DR & BDR in OSPF


DR and BDR Election 
The DR/BDR election occurs during OSPF neighbor ship. Specifically during the last phase of 2-Way neighbor state and just before the ExStart state. All neighbor routers form full adjacencies with the DR and BDR only. The DROther routers will never update other routers in the network.

Facts about DR and BDR

  • The router with the Highest Priority becomes the DR and router with the second highest priority becomes the BDR. 
  • If there is a tie in priority, the router with the Highest Router ID will become DR.
  • By default priority on Cisco routers is 1. We can manually change it 
  • If the Router priority is set to 0 (Zero), that router will not participate in the DR/BDR election. 
  • DR election process is not preemptive
  • If a router with a higher priority is added to the network, it will not become DR until we clear OSPF process and DR/BDR election takes place again