OSPF DR and BDR Configuration

In this lesson, you are going to configure R3 as the Designated Router (DR) and R4 as the Backup Designated Router (BDR) on a shared Ethernet segment.
Then you will break the DR on purpose and watch who takes its place.

The Four Routers

Figure 1 – Topology used for the configuration of DR and BDR

Our topology includes four routers:

• R1: 192.168.1.1

• R2: 192.168.1.2

• R3: 192.168.1.3

• R4: 192.168.1.4

All routers are connected to the 192.168.1.0/24 subnet through their GigabitEthernet0/0 interface.
No router-id is configured, so each router uses its G0/0 address as Router ID.

OSPF Configuration on All Routers

On each router, enable OSPF and advertise the shared network. Start with R1:

R1# conf t
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)# router ospf 1
R1(config-router)# network 192.168.1.0 0.0.0.255 area 0
R1(config-router)# end

Repeat the exact same three commands on R2, R3 and R4.
The network statement includes G0/0 in OSPF and immediately triggers the DR/BDR election.

At this stage, you have not configured any OSPF priority.
All routers are using the default priority of 1.

Reading the Neighbor Table

On R1, check which roles the election handed out:

R1# show ip ospf neighbor

Neighbor ID      Pri  State          Dead Time   Address      Interface
192.168.1.3       1   FULL/BDR       00:00:23    192.168.1.3  GigabitEthernet0/0
192.168.1.4       1   FULL/DR        00:00:23    192.168.1.4  GigabitEthernet0/0
192.168.1.2       1   2WAY/DROTHER   00:00:23    192.168.1.2  GigabitEthernet0/0

Neighbor ID 192.168.1.4, which is R4, is seen as the DR. Neighbor ID 192.168.1.3, which is R3, is seen as the BDR.
R2 stays in 2WAY with R1, because both are DROTHERs.

Why R4 Won

When priorities are equal, OSPF uses the highest Router ID to elect the DR.
The router with the second-highest Router ID becomes the BDR.

• R4 has the highest Router ID, 192.168.1.4, and is elected DR

• R3 has the next highest Router ID, 192.168.1.3, and is elected BDR

This is not the result you want. R3 should be the DR.

To make R3 the new DR, you raise its OSPF priority above the others.
Then you will discover that this alone is not enough.