DR and BDR: a spokesperson for the segment
Why broadcast networks elect a DR, why the BDR matters and why 2-Way can be completely healthy.
Why elect anybody?
On a shared broadcast segment, building a full adjacency between every pair scales poorly. OSPF elects a designated router and a backup. Other routers form full adjacencies with these two.
The election
Among eligible candidates, higher interface priority wins; router ID breaks a tie. Priority zero makes an interface ineligible. Existing DR/BDR roles are non-preemptive: a new router with a higher priority does not simply take over.
When the DR disappears
The BDR can take the DR role and a new BDR is elected. Do not assume the highest configured priority is always today’s DR: election history matters.
What the DR advertises
The DR originates the Type 2 Network LSA for the transit network when appropriate adjacencies exist. It describes the routers attached to that network. It does not become the mandatory forwarding path for everyone’s user traffic.
A useful lab
Put three routers on one broadcast segment. Check the DR/BDR roles and neighbor states. Add a fourth eligible router with a higher priority and observe that the current DR remains. Avoid resetting a production process just to force an election.
Protocol reference: RFC 2328 (OSPFv2). NSSA reference: RFC 3101.