OSPF: DR and BDR Election on a Shared Segment
Priority 255 on the router that should win, and it is still not the DR. Understand why nothing happened, live, right below.
Symptom
The wrong router won the designated router role on a multi-access segment, and raising its priority afterward changed nothing. Find out why.
Your mission
Inspect, recover, then verify
Format
Cisco IOS CLI
Investigate before you configure.
Use the CLI evidence to isolate the fault, make the smallest safe correction, then verify the network state changed.
Observe the symptom and link state.
Diagnose by comparing the protocol evidence.
Verify the expected device state and confirm the original symptom is resolved.
Console access: R1 · R2
This workspace stacks for portrait phones. Rotate to landscape for the full split-screen console.
Need a hint?
Reveal the root cause when you're ready.
Try the investigation first, then use this as your escape hatch, not a dead end.
Lab debrief
Turn the session into a repeatable troubleshooting pattern.
Use this reference after your attempt: first explain the symptom, then verify the evidence, then confirm the repair.
The problem
Three routers share a multi-access segment. A low-powered router became the designated router after a maintenance window, and although its priority has since been lowered and a preferred router raised to 255, the roles have not changed.
What you'll practice
- Explain the DR and BDR election criteria and their order
- Read OSPF interface priority and current DR/BDR state
- Explain why the election is non-preemptive
- Force a re-election safely and verify the resulting roles
1. Explain the DR and BDR election criteria and their order
Highest interface priority wins, and the router ID breaks the tie. A priority of 0 removes a router from the election entirely, which is the cleanest way to keep an unsuitable router out of the role.
2. Read OSPF interface priority and current DR/BDR state
show ip ospf interface names the DR and BDR by router ID and reports the local priority, which is where the discrepancy between intent and reality becomes visible.
3. Explain why the election is non-preemptive
A preemptive election would tear down and rebuild adjacencies every time a router rebooted, so OSPF deliberately leaves an incumbent in place. Understanding this prevents a great deal of wasted troubleshooting.
4. Force a re-election safely and verify the resulting roles
You will use clear ip ospf process and understand its disruptive nature, rather than treating it as a routine command.
The topology
Three routers connect to a common broadcast segment through a switch, all in the same OSPF area, forming adjacencies through an elected designated router.
Commands to run yourself
The real diagnostic commands for this lab, copy-paste ready. Run these in the same order to reproduce the investigation on your own session.
show ip ospf interface GigabitEthernet0/0show ip ospf neighborshow ip protocolsTopology diagram
Fact-checked references
The commands and behaviour in this lab were checked against these primary references.
Frequently asked
Why do two routers sit in 2-WAY instead of FULL?
On a multi-access segment only the DR and BDR form full adjacencies with everyone. Two DROTHER routers stay in 2-WAY with each other by design, which is normal and not a fault.
Is there a way to avoid the election entirely?
Yes. On a point-to-point Ethernet link between exactly two routers, ip ospf network point-to-point skips the DR/BDR election altogether and speeds up convergence.
Ready to run this lab yourself?
No setup, no image sourcing. Book a session or ask for a live demo.