OSPF DR/BDR Election Explained: Using Interface Priority for Better Routing

OSPF DR/BDR Election (Interactive Guide)

In OSPF (Open Shortest Path First), routers on a multi-access network segment—such as Ethernet—elect a Designated Router (DR) and a Backup Designated Router (BDR). This election reduces protocol overhead by limiting the number of adjacencies required on the segment.

While the election process is automatic, network engineers often want to control which routers become the DR or BDR, especially when routers have different roles or capacities.

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How OSPF Influences DR/BDR Selection

OSPF uses an interface priority value to influence DR and BDR elections:

• Higher priority → higher chance of becoming DR or BDR
• Priority 0 → router is excluded from the election
• If priorities tie, the highest Router ID wins

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Example Scenario

Three routers—Router5, Router1, and Router3—share the same Ethernet segment. We want to control the DR/BDR roles explicitly.

Router5 – Designated Router (DR)

Router5# configure terminal
Router5(config)# interface Ethernet0
Router5(config-if)# ip ospf priority 10
Router5(config-if)# end

Router1 – Backup Designated Router (BDR)

Router1# configure terminal
Router1(config)# interface FastEthernet0/0.1
Router1(config-subif)# ip ospf priority 2
Router1(config-subif)# end

Router3 – DROther (Priority 0)

Router3# configure terminal
Router3(config)# interface FastEthernet0/0.1
Router3(config-subif)# ip ospf priority 0
Router3(config-subif)# end

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Interactive DR/BDR Election Topology

Hover over each router to see its OSPF role and priority.

DR BDR DROther

Tip: Changing OSPF priority on a live interface does not trigger a new election. The OSPF process or interface must be reset.

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Key Considerations Across Software Releases

• Dynamic Reelections: Priority changes require a process or interface reset.
• Subinterfaces: Priority can be set per subinterface, but VLAN correctness is critical.
• Interface Types: DR/BDR applies only to multi-access networks.

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Summary

Controlling OSPF DR/BDR elections is a powerful technique for improving network stability and predictability. By assigning priorities intentionally, engineers can ensure that the most capable routers handle adjacency management and LSA flooding.

For a deeper dive into OSPF architecture and behavior, see the OSPF article on Wikipedia .