There is no possibility of more than one DR election in a segment at a time. Ofcourse at a same time incase if DR goes down as and when it is elected BDR will be promoted if available or else there will be new election but other than I do not think there is any chnace.
Still I am not clear, Consider a new segment is formed with 4 routers, since all the 4 routers are new, there is no DR/BDR.In that case, won't there be more than one election happens ? Won't every new router start the election or who will start the election at that time ? It would be better if u explain with an example.
When all the 4 routers come up on the segment, they first send hellos to each other to form neighbor adjacancey with DR field set to 0 in the hello packet.When the 2ways- state is done and router A sees itself as neighbor in router B's hello packet they start the DR/BDR process.The router with the highest priority becomes the DR, if there is a tie router with the higest RID ( router ID) becomes the DR.If loopback IP is configured, highest loopback ip will be the RID or if loopback not configured, the highest IP will become the RID. If a router interface is set with a 0 priority, it will not elected as DR/BDR.
Now, initially BDR is elected and if there is not DR on the segment it is elected as DR for that segment. DR will form adjacency with DR-OTHER and BDR only.For any new router on the segemnt will come to know about the DR and will go to the DR-OTHER state as there is DR preemption on the segment, although it has higher priority then the DR. If you active DR goes down, BDR will be promoted as DR and BDR election will take place.
Thanks for you explanation on DR election.From your explnation, what happens when Router C and D also forms adjacency at the same time of Router A and B, where all are in same segment.In that case, why not Router D or C will have triggered election at the same time ?
if the routers partcipate in the election, the process described by Amit will take place. If they form an adjacency AFTER the DR has been elected, nothing happens. OSPF does not have preemption, that is, the DR will stay the DR, even if other routers with a higher priority (or router ID) come online and form adjacencies. The election only takes place when the DR goes down...
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