4.4.3. Broadcast Networks (DR/BDR Selection)

💡 First Principle: On a broadcast network with 10 routers, full mesh adjacencies would mean 45 pairs exchanging LSAs—that's an explosion of traffic. The DR acts as a central point: all routers send updates to the DR, and the DR re-floods to everyone else. This reduces adjacencies from n² to n.

Election Process: held once routers reach 2-Way; routers with priority 0 are excluded first.

1. Highest OSPF priority wins (default 1; priority 0 = can't be DR/BDR)
2. If tied, highest Router ID wins

Critical rule: DR/BDR election is NON-PREEMPTIVE. If the current DR is running and you add a router with higher priority, it does NOT take over. The new router becomes the BDR if that role is empty, otherwise a DROTHER. Only when the DR fails does the BDR promote to DR, and a new BDR election occurs. This prevents unnecessary re-convergence.

Multicast addresses used:

• 224.0.0.5 (AllSPFRouters) — the DR floods updates to this; EVERY OSPF router listens
• 224.0.0.6 (AllDRouters) — DROTHERs send their updates to this; only the DR and BDR listen

Getting these the wrong way round is a classic exam trap: traffic flows up to the DR on .6 and back down to everyone on .5.

Configuration:

Router(config)# interface GigabitEthernet0/0
Router(config-if)# ip ospf priority 255    ! Make this router DR
Router(config-if)# ip ospf priority 0      ! Prevent DR/BDR role

Verification:

Router# show ip ospf neighbor
Router# show ip ospf interface GigabitEthernet0/0 | include DR

⚠️ Exam Trap: DR/BDR elections happen per network segment, not per router. A router can be DR on one interface and DROTHER on another. Also, point-to-point links don't elect DR/BDR at all—there are only two routers, so it's unnecessary.