3.4OSPF DR/BDR election and router ID

As the previous section showed, broadcast links elect a DR/BDR. CCNA frequently poses diagnostics that require precisely understanding the election rules: "why did the most powerful router not become the DR?" or "why did the router ID not change after I added a loopback?" This section nails down—as real device behavior—that the election is decided by priority then router ID, that a once-elected DR sits tight non-preemptively, and the order in which the router ID is determined.

3.4.1The role of the DR/BDR and the election rules

• A DR (Designated Router) and BDR (Backup DR) exist because, on one broadcast segment, having every router form a Full adjacency with every other would explode the adjacency count to n(n-1)/2. To prevent this, one DR is elected as a representative, and all DROTHERs form Full only with the DR and BDR. The BDR is a standby that takes over instantly if the DR fails.
• The election works as: (1) the router with the highest interface priority (ip ospf priority, default 1, range 0-255) becomes DR, the runner-up BDR. On a priority tie, (2) the router with the highest router ID wins. A router with priority 0 never becomes DR or BDR (always a DROTHER).
• The election is non-preemptive: once a DR is chosen, a later-joining router with a higher priority does not seize the DR role. No changeover occurs until the existing DR fails or OSPF is reset (clear ip ospf process). This contrasts with HSRP's preempt (next section).

3.4.2The router-ID selection order

• The router ID (32 bits, in IP-address form) is chosen in this order: (1) the value set manually with the router-id command -> (2) the highest IP among loopback interfaces -> (3) the highest IP among active physical interfaces. A loopback outranks a physical interface and rarely goes down, yielding a stable router ID.
• The router ID is fixed once, at OSPF process startup. Adding a loopback or changing router-id during operation does not change the existing router ID until you restart OSPF with clear ip ospf process (or reload the router).

Suppose you later added a high-performance new router, R3, to a broadcast segment. You raised R3's interface priority from the default 1 to 100, thinking "this will make R3 the DR," yet show ip ospf neighbor shows R1—which came up earlier—still the DR. Suspecting "priority 100 is not taking effect" and questioning the config would be hasty; the real cause is that OSPF's DR election is non-preemptive. The DR is fixed by the members present when the election first completed on the segment, and a later-joining higher-priority R3 does not seize the existing DR (R1)—this is correct behavior per spec. To deliberately make R3 the DR, you must force a re-election with clear ip ospf process on the segment (done during a maintenance window). Another common case: aiming for a stable router ID, you later configure a loopback interface (with a high IP) on R3, yet show ip ospf's router ID remains the physical interface's IP. This too follows the spec that the router ID is fixed once at process startup and does not change after you add a loopback until you clear ip ospf process—not a misconfiguration. Both DR election and router ID share the pitfall that reflecting a change requires restarting the OSPF process, and knowing this avoids the misdiagnosis of "not reflected = broken."

3.4.3Section summary

• Broadcast networks elect a DR/BDR, and DROTHERs form Full only with them (preventing an adjacency explosion)
• The election is priority (higher wins, 0 ineligible) -> router ID (higher wins), and is non-preemptive (a later higher priority does not take over)
• The router ID is chosen manual -> highest loopback IP -> highest physical IP, fixed at process startup and unchanged until clear ip ospf process