Neighbor priority is different than the interface priority and is NOT used in DR/BDR elections.
Neighbor priority allows preconfigured neighbors to be brought up first, allowing them to establish DR/BDRs first so that routers not intended to become DR/BDR are excluded automatically, and allows the election process to be done only for the intended routers that need to be DR/BDR.
If all neighbors omit the priority setting the the election optimization is not used and all neighbors are contacted at the same time immediately.
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