OSPFCCNA 200-301

OSPF DR/BDR Election Choosing Wrong Router

Presenting Symptom

The OSPF DR/BDR election results in a router with lower priority or lower router ID becoming the DR, causing suboptimal routing and potential instability.

Network Context

A small branch office with three Cisco routers (R1, R2, R3) connected via a broadcast multi-access Ethernet segment (VLAN 10). All routers run IOS 15.x and OSPF in area 0. The network expects R1 (with highest priority) to be DR, but R2 (with lower priority) is elected as DR.

Diagnostic Steps

Check OSPF neighbors and DR/BDR roles

show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface
2.2.2.2          1   FULL/DR         00:00:34    10.0.0.2        GigabitEthernet0/0
3.3.3.3          1   FULL/BDR        00:00:38    10.0.0.3        GigabitEthernet0/0
1.1.1.1          1   FULL/DROTHER    00:00:36    10.0.0.1        GigabitEthernet0/0

The output shows R2 (2.2.2.2) as DR and R3 (3.3.3.3) as BDR, while R1 (1.1.1.1) is DROTHER. This indicates R1 did not become DR despite expected higher priority.

Verify OSPF interface priority on all routers

show ip ospf interface gigabitEthernet 0/0

GigabitEthernet0/0 is up, line protocol is up
  Internet Address 10.0.0.1/24, Area 0
  Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DROTHER, Priority 1
  Designated Router (ID) 2.2.2.2, Interface address 10.0.0.2
  Backup Designated router (ID) 3.3.3.3, Interface address 10.0.0.3

R1 shows priority 1, which is the default. If R1 was intended to be DR, its priority should be higher (e.g., 255). The output confirms R1's priority is not set higher.

Check router IDs and priorities on R2 and R3

show ip ospf interface gigabitEthernet 0/0

GigabitEthernet0/0 is up, line protocol is up
  Internet Address 10.0.0.2/24, Area 0
  Process ID 1, Router ID 2.2.2.2, Network Type BROADCAST, Cost: 1
  Transmit Delay is 1 sec, State DR, Priority 1
  Designated Router (ID) 2.2.2.2, Interface address 10.0.0.2
  Backup Designated router (ID) 3.3.3.3, Interface address 10.0.0.3

R2 also has priority 1, but its router ID (2.2.2.2) is higher than R1's (1.1.1.1). Since all priorities are equal, the highest router ID wins. R2's higher router ID caused it to become DR.

Confirm OSPF process and router ID configuration on R1

show running-config | section router ospf

router ospf 1
 router-id 1.1.1.1
 network 10.0.0.0 0.0.0.255 area 0

R1 has a manually configured router ID 1.1.1.1, but no priority set on the interface. The router ID is lower than R2's, so without priority differentiation, R2 wins the DR election.

Root Cause

The OSPF DR/BDR election on the broadcast segment elected R2 as DR because all routers have the same default interface priority (1), and R2 has the highest router ID (2.2.2.2). R1, which was intended to be DR, has a lower router ID (1.1.1.1) and no priority configured. The election is deterministic based on priority first, then router ID, but since priorities are equal, the highest router ID wins.

Resolution

To make R1 the DR, set its OSPF interface priority to a higher value (e.g., 255) on the relevant interface. Then, reset the OSPF process on all routers to trigger a new election. On R1: interface GigabitEthernet0/0 ip ospf priority 255 On all routers (R1, R2, R3): clear ip ospf process Explanation: The 'ip ospf priority' command sets the interface priority (0-255, default 1). A higher priority increases the chance of becoming DR. After changing priority, the OSPF process must be cleared to force a new election.

Verification

After applying the fix, run 'show ip ospf neighbor' on any router to confirm R1 is now DR: Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 255 FULL/DR 00:00:32 10.0.0.1 GigabitEthernet0/0 3.3.3.3 1 FULL/BDR 00:00:36 10.0.0.3 GigabitEthernet0/0 2.2.2.2 1 FULL/DROTHER 00:00:34 10.0.0.2 GigabitEthernet0/0 R1 now shows as DR with priority 255, and R2 is DROTHER.

Prevention

["Always configure OSPF interface priority on the desired DR router to a value higher than other routers (e.g., 255) to ensure it wins the election.","Avoid relying solely on router ID for DR election; use priority to explicitly control the DR/BDR selection.","When adding a new router to a broadcast segment, consider its router ID and priority to prevent unexpected DR changes."]

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario tests OSPF DR/BDR election mechanics. Questions may present a troubleshooting scenario where a router with a lower router ID becomes DR, and you must identify the cause (equal priorities) and solution (set higher priority). Expect multiple-choice or drag-and-drop questions asking for the correct command to influence the election.

Exam Tips

Remember that OSPF DR/BDR election is non-preemptive; once elected, they stay until OSPF process reset or interface failure.

The election order: highest interface priority (0-255, 0 means never DR/BDR), then highest router ID.

Know the command 'ip ospf priority <0-255>' and 'clear ip ospf process' to force re-election.

Commands Used in This Scenario

clear ip ospf process

Resets the OSPF process on the router, forcing it to re-establish all neighbor adjacencies and re-learn routes.

show ip ospf interface

Displays OSPF interface parameters and status, used to verify OSPF configuration and troubleshoot neighbor relationships.

show ip ospf neighbor

Displays OSPF neighbor information to verify adjacency formation and troubleshoot OSPF neighbor relationships.

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