OSPF DR Election: The Wrong Router Won
A DR election won by the least capable router on the segment, and a fix that needs two steps. Diagnose it and fix it yourself, live, right below.
Symptom
The least capable router on the segment is the Designated Router, and raising the intended router's priority changes nothing at all. There is a second half to this fix that most people miss.
Your mission
Prove the cause, then fix it
Format
Guided CLI investigation
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 · R3
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
R1, R2 and R3 share a broadcast segment in OSPF area 0. R1 is the most capable router and was intended to be the Designated Router, but R3 holds the role and R1 is a DROTHER, so LSA flooding runs through the least capable device.
What you'll practice
- Read DR and BDR roles from the neighbor and interface views
- Understand that priority decides first and router ID only breaks ties
- Know that DR election is non-preemptive
- Force a re-election safely and confirm the new roles
1. Priority first, router ID only as a tiebreak
Every router on the segment was left at the default priority of 1, so the election fell through to the highest router ID and R3 won on 3.3.3.3. You will learn to read both values together rather than assuming the biggest router wins.
2. Non-preemptive is the whole lesson
Set R1's priority to 100 and watch nothing happen. A sitting DR keeps the role for as long as the adjacency stays up, which is deliberate: re-electing on every change would churn the flooding topology. You will learn to expect the change to require a reset.
3. Priority 0 is a different statement
Zero does not mean lowest priority, it means never eligible. You will set it by mistake and see the router excluded from the election entirely, which is a useful thing to have done once on a device that does not matter.
The topology
R1, R2 and R3 all connect to the shared 10.0.0.0/24 broadcast segment in OSPF area 0, with router IDs 1.1.1.1, 2.2.2.2 and 3.3.3.3.
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 neighborshow ip ospf interface GigabitEthernet0/0show ip protocolsTopology diagram
Frequently asked
Why did the least capable router become DR?
Because OSPF does not know or care which router is most capable. It compares interface priority first, and when every router is left at the default of 1 the election falls through to the highest router ID. R3's 3.3.3.3 beat the others, and that is the entire selection process.
I set the priority to 100 and nothing changed. Is the command wrong?
No, the command took effect and the new priority is stored. DR election is non-preemptive, so an existing DR keeps the role until the adjacency resets, no matter how good a candidate appears afterwards. This is deliberate, because re-electing on every configuration change would repeatedly disrupt LSA flooding on the segment.
What does priority 0 do?
It makes the router permanently ineligible to be DR or BDR on that interface. It is not simply the lowest priority, it is an opt-out, and it is the correct setting for a router you never want holding the role, such as a small branch device on a segment with two well-connected cores.
Ready to run this lab yourself?
No setup, no image sourcing. Book a session or ask for a live demo.