OSPF — DR/BDR election on a broadcast segment
- Routing
- Intermediate
- 12 min
- 3 steps
- Cisco IOS-style
Three routers share a common broadcast segment under a hub. With default priorities, the highest router-id (R3 = 3.3.3.3) wins DR; R2 = 2.2.2.2 wins BDR. Configure ip ospf priority on R1 to override the election, then verify with show ip ospf neighbor.
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Free · no install · no account
Scenario
On a broadcast (or NBMA) OSPF segment, every router doesn't peer with every other router — that scales O(n²). Instead, OSPF elects one router as DR (Designated Router) and one as BDR (Backup). All other routers form full adjacencies with DR + BDR only; non-DR-other relationships stay 2-Way. The DR originates a Type-2 Network LSA that enumerates every attached router on the segment, so SPF can traverse it as a transit pseudo-node. Election rules (RFC 2328 §9.4): 1. Priority 0 → never DR/BDR. 2. Highest priority wins. 3. Tie-break by highest router-id. In this lab three routers share a hub-and-spoke broadcast segment. Default priority is 1 everywhere; router-ids are 1.1.1.1, 2.2.2.2, 3.3.3.3. R3 wins DR by router-id; R2 wins BDR. You'll observe this, then override with ip ospf priority on R1 to take DR.
Topology
1 × switch, 3 × router · Cisco IOS-style
hub— switchR1— routerR2— routerR3— router
What you will do
On
R1, runshow ip ospf neighbor. Confirm R3 (highest router-id) is DR and R2 is BDR; R1 sees both as full adjacencies.On
R1, raise the OSPF priority on Gi0/0 to 100 withip ospf priority 100. R1 should now win DR over R3.On
R2, setip ospf priority 0. R2 is now never-DR/BDR — confirm withshow ip ospf neighbor(R2 stays DR-Other regardless of router-id).
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This lab in other languages
OSPF — DR/BDR-Wahl auf einem Broadcast-SegmentOSPF — elección de DR/BDR en un segmento broadcastOSPF — eleição de DR/BDR em um segmento broadcastOSPF — élection DR/BDR sur un segment broadcastOSPF — broadcast segment पर DR/BDR electionOSPF — 广播段上的 DR/BDR 选举OSPF — elezione DR/BDR su un segmento broadcastOSPF — ブロードキャストセグメントでの DR/BDR 選出OSPF — 브로드캐스트 세그먼트에서의 DR/BDR 선출OSPF — выборы DR/BDR в широковещательном сегментеOSPF — yayın segmentinde DR/BDR seçimiOSPF — bầu chọn DR/BDR trên segment broadcast