Does a router with a higher OSPF priority always become DR?

No. Priority matters when an election occurs, but the election is non-preemptive. A new router does not replace a working DR only because it has a higher priority.

OSPF DR/BDR Election and Troubleshooting

Q: Where does OSPF elect a DR and BDR?

OSPF elects a DR and BDR on multi-access network types such as broadcast Ethernet and NBMA. Point-to-point links do not use a DR or BDR.

Q: What does OSPF priority 0 mean?

Priority 0 makes an interface ineligible to become DR or BDR. The router can still participate in OSPF and form full adjacencies with the DR and BDR.

Q: Is a 2-Way state between two DROTHER routers a failure?

No. On a broadcast segment, two DROTHER routers normally remain in the 2-Way state with each other while each forms a Full adjacency with the DR and BDR.

Q: Which router creates the Type 2 Network LSA?

The DR creates the Type 2 Network LSA for the shared multi-access segment. It lists the routers attached to that segment.

DR and BDR at a Glance

The Designated Router (DR) coordinates LSA exchange on a multi-access segment. The Backup Designated Router (BDR) is ready to take over if the DR fails. Other routers on the segment are called DROTHERs.

Election first choiceHighest priority

Tie-breakerHighest router ID

Ineligible valuePriority 0

Important behaviorNon-preemptive

First check: Confirm the OSPF network type. Broadcast and NBMA networks use an election. Point-to-point links do not.

1. See the Broadcast-Segment Design

All four routers below share one Ethernet LAN in Area 0. Their interface priorities make the intended roles easy to predict.

10.10.10.0/24Shared Ethernet segment · OSPF Area 0

DRR1Priority 200 · RID 1.1.1.110.10.10.1

BDRR2Priority 100 · RID 2.2.2.210.10.10.2

DROTHERR3Priority 1 · RID 3.3.3.310.10.10.3

INELIGIBLER4Priority 0 · RID 4.4.4.410.10.10.4

Expected result after a fresh election: R1 is DR, R2 is BDR, R3 is DROTHER, and R4 cannot become DR or BDR.

Do not start with the router ID: OSPF compares interface priority first. Router ID matters only when eligible routers have the same priority.

2. Understand Why OSPF Uses an Election

A broadcast LAN can contain many routers. If every router built a Full adjacency with every other router, the number of relationships would grow quickly. With four routers, a full mesh needs six relationships. With ten routers, it needs 45.

1

DROTHER sends updates

DROTHER routers send link-state updates to 224.0.0.6, which reaches the DR and BDR.

2

DR coordinates flooding

The DR repeats the update to all OSPF routers on 224.0.0.5.

3

DR represents the LAN

The DR creates the Type 2 Network LSA for the shared segment.

The BDR listens and stays synchronized so it can replace the DR quickly. This design reduces unnecessary Full adjacencies while keeping the segment resilient.

3. Apply the Election Rules in Order

Remove priority 0 interfaces. They can join OSPF but cannot be DR or BDR.
Compare interface priority. The highest eligible value wins. The range is 0–255 and the default is 1.
Use router ID only for a tie. The numerically highest router ID wins between equal priorities.
Elect the BDR before the DR. OSPF first selects the best BDR candidate, then promotes the existing or selected DR candidate.
Keep existing roles. A working DR is not replaced just because a better candidate joins later.

Router	Priority	Router ID	Expected role	Reason
R1	200	1.1.1.1	DR	Highest eligible priority
R2	100	2.2.2.2	BDR	Second-highest priority
R3	1	3.3.3.3	DROTHER	Eligible, but lower priority
R4	0	4.4.4.4	Ineligible	Priority 0 overrides its high RID

4. Configure a Predictable Election

Set stable router IDs under the OSPF process and set election priority on the interface connected to the shared LAN. Priority is an interface setting, not a router-wide setting.

R1 · Preferred DR

router ospf 1
 router-id 1.1.1.1
!
interface GigabitEthernet0/0
 ip ospf 1 area 0
 ip ospf priority 200

R2 · Preferred BDR

router ospf 1
 router-id 2.2.2.2
!
interface GigabitEthernet0/0
 ip ospf 1 area 0
 ip ospf priority 100

R3 · DROTHER

router ospf 1
 router-id 3.3.3.3
!
interface GigabitEthernet0/0
 ip ospf 1 area 0
 ip ospf priority 1

R4 · Never DR/BDR

router ospf 1
 router-id 4.4.4.4
!
interface GigabitEthernet0/0
 ip ospf 1 area 0
 ip ospf priority 0

Production habit: Configure priorities before routers form neighbors. This avoids a disruptive election reset later.

5. Understand FULL and 2-WAY States

Not every pair of routers on a broadcast LAN must reach FULL. The role of each neighbor decides the expected state.

Neighbor pair	Expected state	Meaning
DR ↔ BDR	FULL	The backup keeps a complete synchronized database.
DR or BDR ↔ DROTHER	FULL	Each DROTHER exchanges its database with both central routers.
DROTHER ↔ DROTHER	2-WAY	They see each other but do not build a Full adjacency.

Common exam trap: A stable 2-WAY state between two DROTHER routers on Ethernet is normal. Do not troubleshoot it as a failure.

6. Verify Roles and the Type 2 LSA

Read the neighbor table first, confirm the local interface state second, and inspect the LSDB last. This order moves from the visible result to the supporting evidence.

R3 neighbor view

R3# show ip ospf neighbor

Neighbor ID  Pri  State       Address
1.1.1.1      200  FULL/DR     10.10.10.1
2.2.2.2      100  FULL/BDR    10.10.10.2
4.4.4.4        0  2WAY/DROTHER 10.10.10.4

Check the local interface

R1# show ip ospf interface g0/0
Process ID 1, Router ID 1.1.1.1
Network Type BROADCAST, Cost: 1
State DR, Priority 200
Designated Router (ID) 1.1.1.1
Backup Designated Router (ID) 2.2.2.2

Confirm the Network LSA

R1# show ip ospf database network

Link ID         ADV Router      Age
10.10.10.1      1.1.1.1         42

Attached Router 1.1.1.1
Attached Router 2.2.2.2
Attached Router 3.3.3.3
Attached Router 4.4.4.4

show ip ospf neighbor proves each neighbor role and adjacency state.
show ip ospf interface g0/0 proves network type, priority, DR and BDR.
show ip ospf database network proves the DR originated a Type 2 LSA.

7. Change an Election Safely

OSPF DR elections are non-preemptive. If R3 joins with priority 250 after R1 is already DR, R1 stays DR. R3 can win only when a new election is required.

1

Current DR fails

Neighbors stop receiving Hellos from the DR and the dead timer expires.

2

BDR becomes DR

The existing BDR takes over without waiting for a completely new choice.

3

New BDR is elected

The remaining eligible routers compare priority and then router ID.

Use care: Clearing an OSPF process or bouncing an interface disrupts neighbors and routes. Do it only in an isolated lab or an approved maintenance window.

8. Troubleshoot Unexpected Results

If the result does not match your design, check facts in this order. Avoid resetting OSPF until you understand the cause.

Network type: All interfaces on the segment should agree. Point-to-point mode has no election.
Interface priority: Check the actual connected interface, not another interface on the router.
Router ID: Confirm it is unique and remember that it is only a tie-breaker.
Election timing: A better candidate that joined later will not preempt the current DR.
Neighbor state: FULL with DR/BDR and 2-WAY between DROTHERs is normal.
Duplicate router IDs: A duplicate can break stable adjacency and produce confusing output.

9. Guided Practice Lab

Complete one task at a time. Predict the result before running the command, then compare your prediction with the router output.

Task 1: Build and Predict

Connect four routers to one Ethernet switch.
Use the addresses and priorities shown in the topology.
Enable OSPF Area 0 on every LAN interface.
Write down the expected DR, BDR and DROTHER roles.
Verify with show ip ospf neighbor.

Expected result: R1 is DR, R2 is BDR, and your prediction matches the output.

Task 2: Prove Two Important Rules

Confirm R4 uses priority 0.
Prove R4 still reaches FULL with R1 and R2.
Compare the R3-to-R4 relationship.
Explain why their 2-WAY state is correct.

Expected result: Priority 0 blocks election eligibility, not OSPF participation.

Task 3: Observe DR Failure

Save the initial neighbor and interface output.
In an isolated lab, shut R1's LAN interface.
Watch R2 move from BDR to DR.
Identify the newly elected BDR.
Restore R1 and confirm it does not automatically reclaim DR.

Expected result: You can explain BDR takeover and non-preemption using captured output.

10. Frequently Asked Questions

Where does OSPF elect a DR and BDR?

OSPF uses an election on multi-access network types such as broadcast Ethernet and NBMA. A point-to-point link has only two OSPF speakers, so it does not need a DR or BDR.

Does a router with a higher priority always become DR?

No. Priority decides an election, but a new router does not replace a working DR. The design is non-preemptive.

What does OSPF priority 0 mean?

It makes that interface ineligible to become DR or BDR. The router can still exchange OSPF traffic and form Full adjacencies with the DR and BDR.

Is a 2-WAY state between two DROTHER routers a failure?

No. On a broadcast network, two DROTHER routers normally stay in 2-WAY with each other. Each one should be FULL with the DR and BDR.

Which router creates the Type 2 Network LSA?

The DR creates the Type 2 Network LSA. It represents the shared segment and lists the routers attached to it.

OSPF DR and BDR Election