OSPF Series 3: DR-BDR Election Process with Flowchart

DR and BDR Selection Process and Flowchart

• The DR and BDR happen for each segment of the network (not for each area). SO, it means one interface of a router can be a DR but another interface of the same router can be a DRO
• The flowchart of selecting the DR and BDR is given below:

(NOTE 1: Note that this process takes place locally in each and every router after the adjacency is formed, so some routers may go for the State Machine again if their OWN state changes while others may not go for it again. But, in end, all of them will have a common DR and BDR selected)

(NOTE 2:

• If a router is unable to find any neighbors, then, after its Interface Waiting state is timed out, it automatically elects itself as the DR. In this case, its Hello packet will have its own Router ID as DR and there will be no BDR.
• BUT, if before its Interface Waiting State is timed out, it receives a hello from the BDR router itself, then, the router moves from Waiting to BackupSeen state and copies the DR and BDR information from the hello packet of BDR router because if a BDR is present, then, definitely a DR is also already elected. If the router receives hello packet from the DR router itself, then, it copies only the DR information and waits for a hello from the BDR router. If no hello from BDR router is received before Waiting timeout, then this router decides that there is no BDR elected and starts a BDR election process and it can also become the BDR. This is because a presence of DR does not imply presence of BDR

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